diff --git a/core/lib/Drupal/Core/Path/AliasStorageInterface.php b/core/lib/Drupal/Core/Path/AliasStorageInterface.php
index 23a7619..ae79300 100644
--- a/core/lib/Drupal/Core/Path/AliasStorageInterface.php
+++ b/core/lib/Drupal/Core/Path/AliasStorageInterface.php
@@ -29,8 +29,8 @@
    * @return array|false
    *   FALSE if the path could not be saved or an associative array containing
    *   the following keys:
-   *   - source (string): The internal system path.
-   *   - alias (string): The URL alias.
+   *   - source (string): The internal system path with a starting slash.
+   *   - alias (string): The URL alias with a starting slash.
    *   - pid (int): Unique path alias identifier.
    *   - langcode (string): The language code of the alias.
    *   - original: For updates, an array with source, alias and langcode with
@@ -50,8 +50,8 @@ public function save($source, $alias, $langcode = LanguageInterface::LANGCODE_NO
    * @return array|false
    *   FALSE if no alias was found or an associative array containing the
    *   following keys:
-   *   - source (string): The internal system path.
-   *   - alias (string): The URL alias.
+   *   - source (string): The internal system path with a starting lash.
+   *   - alias (string): The URL alias with a starting slash.
    *   - pid (int): Unique path alias identifier.
    *   - langcode (string): The language code of the alias.
    */
diff --git a/core/lib/Drupal/Core/Path/PathMatcher.php b/core/lib/Drupal/Core/Path/PathMatcher.php
index 10a3899..1f7b88c 100644
--- a/core/lib/Drupal/Core/Path/PathMatcher.php
+++ b/core/lib/Drupal/Core/Path/PathMatcher.php
@@ -73,7 +73,7 @@ public function matchPath($path, $patterns) {
       // Convert path settings to a regular expression.
       $to_replace = array(
         // Replace newlines with a logical 'or'.
-        '/\/(\r\n?|\n)/',
+        '/(\r\n?|\n)/',
         // Quote asterisks.
         '/\\\\\*/',
         // Quote <front> keyword.
diff --git a/core/lib/Drupal/Core/PathProcessor/PathProcessorFront.php b/core/lib/Drupal/Core/PathProcessor/PathProcessorFront.php
index 5be2387..024babf 100644
--- a/core/lib/Drupal/Core/PathProcessor/PathProcessorFront.php
+++ b/core/lib/Drupal/Core/PathProcessor/PathProcessorFront.php
@@ -50,7 +50,7 @@ public function processInbound($path, Request $request) {
    */
   public function processOutbound($path, &$options = array(), Request $request = NULL, CacheableMetadata $cacheable_metadata = NULL) {
     // The special path '<front>' links to the default front page.
-    if ($path == '<front>') {
+    if ($path == '/<front>') {
       $path = '/';
     }
     return $path;
diff --git a/core/modules/language/src/Tests/LanguageSwitchingTest.php b/core/modules/language/src/Tests/LanguageSwitchingTest.php
index cc6068a..c9ff8d0 100644
--- a/core/modules/language/src/Tests/LanguageSwitchingTest.php
+++ b/core/modules/language/src/Tests/LanguageSwitchingTest.php
@@ -37,7 +37,7 @@ protected function setUp() {
   /**
    * Functional tests for the language switcher block.
    */
-  function ptestLanguageBlock() {
+  function testLanguageBlock() {
     // Add language.
     $edit = array(
       'predefined_langcode' => 'fr',
@@ -163,7 +163,7 @@ protected function doTestLanguageBlockAnonymous($block_label) {
   /**
    * Test language switcher links for domain based negotiation.
    */
-  function ptestLanguageBlockWithDomain() {
+  function testLanguageBlockWithDomain() {
     // Add the Italian language.
     ConfigurableLanguage::createFromLangcode('it')->save();
 
@@ -225,7 +225,7 @@ function ptestLanguageBlockWithDomain() {
   /**
    * Test active class on links when switching languages.
    */
-  function ptestLanguageLinkActiveClass() {
+  function testLanguageLinkActiveClass() {
     // Add language.
     $edit = array(
       'predefined_langcode' => 'fr',
diff --git a/core/modules/migrate/config/schema/migrate.data_types.schema.yml b/core/modules/migrate/config/schema/migrate.data_types.schema.yml
index edb628a..d887785 100644
--- a/core/modules/migrate/config/schema/migrate.data_types.schema.yml
+++ b/core/modules/migrate/config/schema/migrate.data_types.schema.yml
@@ -14,6 +14,10 @@ migrate_destination:
 migrate_source:
   type: migrate_plugin
   label: 'Source'
+  mapping:
+    constants:
+      type: ignore
+      label: 'Constants'
 
 # Base schema for migrate source plugins that extend
 # \Drupal\migrate\Plugin\migrate\source\SqlBase.
diff --git a/core/modules/migrate/config/schema/migrate.source.schema.yml b/core/modules/migrate/config/schema/migrate.source.schema.yml
index d9101a0..f3dae0f 100644
--- a/core/modules/migrate/config/schema/migrate.source.schema.yml
+++ b/core/modules/migrate/config/schema/migrate.source.schema.yml
@@ -11,6 +11,3 @@ migrate.source.empty:
     provider:
       type: string
       label: 'Provider'
-    constants:
-      type: ignore
-      label: 'Constants'
diff --git a/core/modules/migrate_drupal/config/optional/migrate.migration.d6_url_alias.yml b/core/modules/migrate_drupal/config/optional/migrate.migration.d6_url_alias.yml
index a8579e9..454ded8 100644
--- a/core/modules/migrate_drupal/config/optional/migrate.migration.d6_url_alias.yml
+++ b/core/modules/migrate_drupal/config/optional/migrate.migration.d6_url_alias.yml
@@ -4,10 +4,20 @@ migration_tags:
   - Drupal 6
 source:
   plugin: d6_url_alias
+  constants:
+    slash: '/'
 
 process:
-  source: src
-  alias: dst
+  source:
+    plugin: concat
+    source:
+      - constants/slash
+      - src
+  alias:
+    plugin: concat
+    source:
+      - constants/slash
+      - dst
   langcode: language
 
 destination:
diff --git a/core/modules/migrate_drupal/src/Tests/d6/MigrateUrlAliasTest.php b/core/modules/migrate_drupal/src/Tests/d6/MigrateUrlAliasTest.php
index 091f85c..eae71e4 100644
--- a/core/modules/migrate_drupal/src/Tests/d6/MigrateUrlAliasTest.php
+++ b/core/modules/migrate_drupal/src/Tests/d6/MigrateUrlAliasTest.php
@@ -73,7 +73,7 @@ public function testUrlAlias() {
     $executable->import();
 
     $path = \Drupal::service('path.alias_storage')->load(array('pid' => $path['pid']));
-    $this->assertIdentical('new-url-alias', $path['alias']);
+    $this->assertIdentical('/new-url-alias', $path['alias']);
   }
 
 }
diff --git a/core/modules/path/src/Plugin/Field/FieldType/PathItem.php b/core/modules/path/src/Plugin/Field/FieldType/PathItem.php
index c06271d..61f5c5f 100644
--- a/core/modules/path/src/Plugin/Field/FieldType/PathItem.php
+++ b/core/modules/path/src/Plugin/Field/FieldType/PathItem.php
@@ -59,7 +59,7 @@ public function insert() {
     if ($this->alias) {
       $entity = $this->getEntity();
 
-      if ($path = \Drupal::service('path.alias_storage')->save($entity->urlInfo()->getInternalPath(), $this->alias, $this->getLangcode())) {
+      if ($path = \Drupal::service('path.alias_storage')->save('/' . $entity->urlInfo()->getInternalPath(), $this->alias, $this->getLangcode())) {
         $this->pid = $path['pid'];
       }
     }
@@ -76,7 +76,7 @@ public function update() {
     // Only save a non-empty alias.
     elseif ($this->alias) {
       $entity = $this->getEntity();
-      \Drupal::service('path.alias_storage')->save($entity->urlInfo()->getInternalPath(), $this->alias, $this->getLangcode(), $this->pid);
+      \Drupal::service('path.alias_storage')->save('/' . $entity->urlInfo()->getInternalPath(), $this->alias, $this->getLangcode(), $this->pid);
     }
   }
 
@@ -86,7 +86,7 @@ public function update() {
   public function delete() {
     // Delete all aliases associated with this entity.
     $entity = $this->getEntity();
-    \Drupal::service('path.alias_storage')->delete(array('source' => $entity->urlInfo()->getInternalPath()));
+    \Drupal::service('path.alias_storage')->delete(array('source' => '/' . $entity->urlInfo()->getInternalPath()));
   }
 
   /**
diff --git a/core/modules/path/src/Plugin/Field/FieldWidget/PathWidget.php b/core/modules/path/src/Plugin/Field/FieldWidget/PathWidget.php
index 03f15b3..957484c 100644
--- a/core/modules/path/src/Plugin/Field/FieldWidget/PathWidget.php
+++ b/core/modules/path/src/Plugin/Field/FieldWidget/PathWidget.php
@@ -33,7 +33,7 @@ public function formElement(FieldItemListInterface $items, $delta, array $elemen
     $entity = $items->getEntity();
     $path = array();
     if (!$entity->isNew()) {
-      $conditions = array('source' => $entity->urlInfo()->getInternalPath());
+      $conditions = array('source' => '/' . $entity->urlInfo()->getInternalPath());
       if ($items->getLangcode() != LanguageInterface::LANGCODE_NOT_SPECIFIED) {
         $conditions['langcode'] = $items->getLangcode();
       }
@@ -44,7 +44,7 @@ public function formElement(FieldItemListInterface $items, $delta, array $elemen
     }
     $path += array(
       'pid' => NULL,
-      'source' => !$entity->isNew() ? $entity->urlInfo()->getInternalPath() : NULL,
+      'source' => !$entity->isNew() ? '/' . $entity->urlInfo()->getInternalPath() : NULL,
       'alias' => '',
       'langcode' => $items->getLangcode(),
     );
@@ -58,7 +58,7 @@ public function formElement(FieldItemListInterface $items, $delta, array $elemen
       '#default_value' => $path['alias'],
       '#required' => $element['#required'],
       '#maxlength' => 255,
-      '#description' => $this->t('The alternative URL for this content. Use a relative path. For example, enter "about" for the about page.'),
+      '#description' => $this->t('The alternative URL for this content. Use a relative path. For example, enter "/about" for the about page.'),
     );
     $element['pid'] = array(
       '#type' => 'value',
@@ -85,7 +85,7 @@ public function formElement(FieldItemListInterface $items, $delta, array $elemen
    */
   public static function validateFormElement(array &$element, FormStateInterface $form_state) {
     // Trim the submitted value of whitespace and slashes.
-    $alias = trim(trim($element['alias']['#value']), " \\/");
+    $alias = rtrim(trim($element['alias']['#value']), " \\/");
     if (!empty($alias)) {
       $form_state->setValueForElement($element['alias'], $alias);
 
@@ -95,6 +95,10 @@ public static function validateFormElement(array &$element, FormStateInterface $
         $form_state->setError($element, t('The alias is already in use.'));
       }
     }
+
+    if ($alias[0] !== '/') {
+      $form_state->setError($element, t('The alias needs to start with a slash.'));
+    }
   }
 
   /**
diff --git a/core/modules/shortcut/src/Tests/ShortcutLinksTest.php b/core/modules/shortcut/src/Tests/ShortcutLinksTest.php
index 52ec545..f9846fa 100644
--- a/core/modules/shortcut/src/Tests/ShortcutLinksTest.php
+++ b/core/modules/shortcut/src/Tests/ShortcutLinksTest.php
@@ -34,8 +34,8 @@ public function testShortcutLinkAdd() {
 
     // Create an alias for the node so we can test aliases.
     $path = array(
-      'source' => 'node/' . $this->node->id(),
-      'alias' => $this->randomMachineName(8),
+      'source' => '/node/' . $this->node->id(),
+      'alias' => '/' . $this->randomMachineName(8),
     );
     $this->container->get('path.alias_storage')->save($path['source'], $path['alias']);
 
diff --git a/core/modules/system/src/Plugin/Condition/RequestPath.php b/core/modules/system/src/Plugin/Condition/RequestPath.php
index a309c58..876656c 100644
--- a/core/modules/system/src/Plugin/Condition/RequestPath.php
+++ b/core/modules/system/src/Plugin/Condition/RequestPath.php
@@ -153,7 +153,7 @@ public function evaluate() {
 
     $request = $this->requestStack->getCurrentRequest();
     // Compare the lowercase path alias (if any) and internal path.
-    $path = trim($this->currentPath->getPath($request), '/');
+    $path = rtrim($this->currentPath->getPath($request), '/');
     $path_alias = Unicode::strtolower($this->aliasManager->getAliasByPath($path));
 
     return $this->pathMatcher->matchPath($path_alias, $pages) || (($path != $path_alias) && $this->pathMatcher->matchPath($path, $pages));
diff --git a/core/modules/system/src/Tests/Path/AliasTest.php b/core/modules/system/src/Tests/Path/AliasTest.php
index f1f6bf2..006bb51 100644
--- a/core/modules/system/src/Tests/Path/AliasTest.php
+++ b/core/modules/system/src/Tests/Path/AliasTest.php
@@ -51,8 +51,8 @@ function testCRUD() {
     }
 
     // Load alias by source path.
-    $loadedAlias = $aliasStorage->load(array('source' => 'node/1'));
-    $this->assertEqual($loadedAlias['alias'], 'alias_for_node_1_und', format_string('The last created alias loaded by default.'));
+    $loadedAlias = $aliasStorage->load(array('source' => '/node/1'));
+    $this->assertEqual($loadedAlias['alias'], '/alias_for_node_1_und', format_string('The last created alias loaded by default.'));
 
     //Update a few aliases
     foreach ($aliases as $alias) {
@@ -90,8 +90,8 @@ function testLookupPath() {
     // Test the situation where the source is the same for multiple aliases.
     // Start with a language-neutral alias, which we will override.
     $path = array(
-      'source' => "user/1",
-      'alias' => 'foo',
+      'source' => "/user/1",
+      'alias' => '/foo',
     );
 
     $aliasStorage->save($path['source'], $path['alias']);
@@ -100,8 +100,8 @@ function testLookupPath() {
 
     // Create a language specific alias for the default language (English).
     $path = array(
-      'source' => "user/1",
-      'alias' => "users/Dries",
+      'source' => "/user/1",
+      'alias' => "/users/Dries",
       'langcode' => 'en',
     );
     $aliasStorage->save($path['source'], $path['alias'], $path['langcode']);
@@ -112,16 +112,16 @@ function testLookupPath() {
 
     // Create a language-neutral alias for the same path, again.
     $path = array(
-      'source' => "user/1",
-      'alias' => 'bar',
+      'source' => "/user/1",
+      'alias' => '/bar',
     );
     $aliasStorage->save($path['source'], $path['alias']);
     $this->assertEqual($aliasManager->getAliasByPath($path['source']), "users/Dries", 'English alias still returned after entering a language-neutral alias.');
 
     // Create a language-specific (xx-lolspeak) alias for the same path.
     $path = array(
-      'source' => "user/1",
-      'alias' => 'LOL',
+      'source' => "/user/1",
+      'alias' => '/LOL',
       'langcode' => 'xx-lolspeak',
     );
     $aliasStorage->save($path['source'], $path['alias'], $path['langcode']);
@@ -132,8 +132,8 @@ function testLookupPath() {
     // Create a new alias for this path in English, which should override the
     // previous alias for "user/1".
     $path = array(
-      'source' => "user/1",
-      'alias' => 'users/my-new-path',
+      'source' => "/user/1",
+      'alias' => '/users/my-new-path',
       'langcode' => 'en',
     );
     $aliasStorage->save($path['source'], $path['alias'], $path['langcode']);
@@ -151,10 +151,10 @@ function testLookupPath() {
 
     // Test the situation where the alias and language are the same, but
     // the source differs. The newer alias record should be returned.
-    $aliasStorage->save('user/2', 'bar');
+    $aliasStorage->save('/user/2', '/bar');
     // Hook that clears cache is not executed with unit tests.
     $aliasManager->cacheClear();
-    $this->assertEqual($aliasManager->getPathByAlias('bar'), 'user/2', 'Newer alias record is returned when comparing two LanguageInterface::LANGCODE_NOT_SPECIFIED paths with the same alias.');
+    $this->assertEqual($aliasManager->getPathByAlias('bar'), '/user/2', 'Newer alias record is returned when comparing two LanguageInterface::LANGCODE_NOT_SPECIFIED paths with the same alias.');
   }
 
   /**
@@ -181,21 +181,21 @@ function testWhitelist() {
     $this->assertNull($whitelist->get($this->randomMachineName()));
 
     // Add an alias for user/1, user should get whitelisted now.
-    $aliasStorage->save('user/1', $this->randomMachineName());
+    $aliasStorage->save('/user/1', '/' . $this->randomMachineName());
     $aliasManager->cacheClear();
-    $this->assertTrue($whitelist->get('user'));
-    $this->assertNull($whitelist->get('admin'));
+    $this->assertTrue($whitelist->get('/user'));
+    $this->assertNull($whitelist->get('/admin'));
     $this->assertNull($whitelist->get($this->randomMachineName()));
 
     // Add an alias for admin, both should get whitelisted now.
-    $aliasStorage->save('admin/something', $this->randomMachineName());
+    $aliasStorage->save('/admin/something', '/' . $this->randomMachineName());
     $aliasManager->cacheClear();
-    $this->assertTrue($whitelist->get('user'));
-    $this->assertTrue($whitelist->get('admin'));
+    $this->assertTrue($whitelist->get('/user'));
+    $this->assertTrue($whitelist->get('/admin'));
     $this->assertNull($whitelist->get($this->randomMachineName()));
 
     // Remove the user alias again, whitelist entry should be removed.
-    $aliasStorage->delete(array('source' => 'user/1'));
+    $aliasStorage->delete(array('source' => '/user/1'));
     $aliasManager->cacheClear();
     $this->assertNull($whitelist->get('user'));
     $this->assertTrue($whitelist->get('admin'));
diff --git a/core/modules/system/src/Tests/Path/UrlAliasFixtures.php b/core/modules/system/src/Tests/Path/UrlAliasFixtures.php
index 550ea76..aa65075 100644
--- a/core/modules/system/src/Tests/Path/UrlAliasFixtures.php
+++ b/core/modules/system/src/Tests/Path/UrlAliasFixtures.php
@@ -48,23 +48,23 @@ public function dropTables(Connection $connection) {
   public function sampleUrlAliases() {
     return array(
       array(
-        'source' => 'node/1',
-        'alias' => 'alias_for_node_1_en',
+        'source' => '/node/1',
+        'alias' => '/alias_for_node_1_en',
         'langcode' => 'en'
       ),
       array(
-        'source' => 'node/2',
-        'alias' => 'alias_for_node_2_en',
+        'source' => '/node/2',
+        'alias' => '/alias_for_node_2_en',
         'langcode' => 'en'
       ),
       array(
-        'source' => 'node/1',
-        'alias' => 'alias_for_node_1_fr',
+        'source' => '/node/1',
+        'alias' => '/alias_for_node_1_fr',
         'langcode' => 'fr'
       ),
       array(
-        'source' => 'node/1',
-        'alias' => 'alias_for_node_1_und',
+        'source' => '/node/1',
+        'alias' => '/alias_for_node_1_und',
         'langcode' => 'und'
       )
     );
diff --git a/core/modules/system/src/Tests/Path/UrlAlterFunctionalTest.php b/core/modules/system/src/Tests/Path/UrlAlterFunctionalTest.php
index 1988708..d1cb096 100644
--- a/core/modules/system/src/Tests/Path/UrlAlterFunctionalTest.php
+++ b/core/modules/system/src/Tests/Path/UrlAlterFunctionalTest.php
@@ -40,14 +40,14 @@ function testUrlAlter() {
     $this->assertUrlOutboundAlter("user/$uid", "user/$name");
 
     // Test that a path always uses its alias.
-    $path = array('source' => "user/$uid/test1", 'alias' => 'alias/test1');
+    $path = array('source' => "/user/$uid/test1", 'alias' => '/alias/test1');
     $this->container->get('path.alias_storage')->save($path['source'], $path['alias']);
     $this->rebuildContainer();
     $this->assertUrlInboundAlter('alias/test1', "user/$uid/test1");
     $this->assertUrlOutboundAlter("user/$uid/test1", 'alias/test1');
 
     // Test adding an alias via the UI.
-    $edit = array('source' => "user/$uid/edit", 'alias' => 'alias/test2');
+    $edit = array('source' => "/user/$uid/edit", 'alias' => '/alias/test2');
     $this->drupalPostForm('admin/config/search/path/add', $edit, t('Save'));
     $this->assertText(t('The alias has been saved.'));
     $this->drupalGet('alias/test2');
diff --git a/core/modules/system/src/Tests/Plugin/Condition/RequestPathTest.php b/core/modules/system/src/Tests/Plugin/Condition/RequestPathTest.php
index 45011f8..fae5d1b 100644
--- a/core/modules/system/src/Tests/Plugin/Condition/RequestPathTest.php
+++ b/core/modules/system/src/Tests/Plugin/Condition/RequestPathTest.php
@@ -86,7 +86,7 @@ public function testConditions() {
     // Get the request path condition and test and configure it to check against
     // different patterns and requests.
 
-    $pages = "my/pass/page\r\nmy/pass/page2\r\nfoo";
+    $pages = "/my/pass/page\r\n/my/pass/page2\r\n/foo";
 
     $request = Request::create('/my/pass/page2');
     $this->requestStack->push($request);
@@ -95,42 +95,42 @@ public function testConditions() {
     $condition = $this->pluginManager->createInstance('request_path');
     $condition->setConfig('pages', $pages);
 
-    $this->aliasManager->addAlias('my/pass/page2', 'my/pass/page2');
+    $this->aliasManager->addAlias('/my/pass/page2', '/my/pass/page2');
 
     $this->assertTrue($condition->execute(), 'The request path matches a standard path');
-    $this->assertEqual($condition->summary(), 'Return true on the following pages: my/pass/page, my/pass/page2, foo', 'The condition summary matches for a standard path');
+    $this->assertEqual($condition->summary(), 'Return true on the following pages: /my/pass/page, /my/pass/page2, /foo', 'The condition summary matches for a standard path');
 
     // Test an aliased path.
     $this->currentPath->setPath('/my/aliased/page', $request);
     $this->requestStack->pop();
     $this->requestStack->push($request);
 
-    $this->aliasManager->addAlias('my/aliased/page', 'my/pass/page');
+    $this->aliasManager->addAlias('/my/aliased/page', '/my/pass/page');
 
     $this->assertTrue($condition->execute(), 'The request path matches an aliased path');
-    $this->assertEqual($condition->summary(), 'Return true on the following pages: my/pass/page, my/pass/page2, foo', 'The condition summary matches for an aliased path');
+    $this->assertEqual($condition->summary(), 'Return true on the following pages: /my/pass/page, /my/pass/page2, /foo', 'The condition summary matches for an aliased path');
 
     // Test a wildcard path.
-    $this->aliasManager->addAlias('my/pass/page3', 'my/pass/page3');
+    $this->aliasManager->addAlias('/my/pass/page3', '/my/pass/page3');
     $this->currentPath->setPath('/my/pass/page3', $request);
     $this->requestStack->pop();
     $this->requestStack->push($request);
 
-    $condition->setConfig('pages', 'my/pass/*');
+    $condition->setConfig('pages', '/my/pass/*');
 
     $this->assertTrue($condition->evaluate(), 'The system_path my/pass/page3 passes for wildcard paths.');
-    $this->assertEqual($condition->summary(), 'Return true on the following pages: my/pass/*', 'The condition summary matches for a wildcard path');
+    $this->assertEqual($condition->summary(), 'Return true on the following pages: /my/pass/*', 'The condition summary matches for a wildcard path');
 
     // Test a missing path.
     $this->requestStack->pop();
     $this->requestStack->push($request);
     $this->currentPath->setPath('/my/fail/page4', $request);
 
-    $condition->setConfig('pages', 'my/pass/*');
+    $condition->setConfig('pages', '/my/pass/*');
 
-    $this->aliasManager->addAlias('my/fail/page4', 'my/fail/page4');
+    $this->aliasManager->addAlias('/my/fail/page4', '/my/fail/page4');
 
-    $this->assertFalse($condition->evaluate(), 'The system_path my/pass/page4 fails for a missing path.');
+    $this->assertFalse($condition->evaluate(), 'The system_path /my/pass/page4 fails for a missing path.');
 
   }
 }
diff --git a/core/modules/system/src/Tests/Routing/MockAliasManager.php b/core/modules/system/src/Tests/Routing/MockAliasManager.php
index 6c3839d..a832432 100644
--- a/core/modules/system/src/Tests/Routing/MockAliasManager.php
+++ b/core/modules/system/src/Tests/Routing/MockAliasManager.php
@@ -55,6 +55,13 @@ class MockAliasManager implements AliasManagerInterface {
   public function addAlias($path, $alias, $path_language = NULL) {
     $language = $path_language ?: $this->defaultLanguage;
 
+    if ($path[0] !== '/') {
+      throw new \InvalidArgumentException('The path needs to start with a slash.');
+    }
+    if ($alias[0] !== '/') {
+      throw new \InvalidArgumentException('The alias needs to start with a slash.');
+    }
+
     $this->aliases[$path][$language] = $alias;
     $this->systemPaths[$alias][$language] = $path;
   }
diff --git a/core/tests/Drupal/Tests/Core/Path/AliasManagerTest.php b/core/tests/Drupal/Tests/Core/Path/AliasManagerTest.php
index 67bfed0..dfc7d63 100644
--- a/core/tests/Drupal/Tests/Core/Path/AliasManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Path/AliasManagerTest.php
@@ -88,7 +88,7 @@ protected function setUp() {
    * @covers ::getPathByAlias
    */
   public function testGetPathByAliasNoMatch() {
-    $alias = $this->randomMachineName();
+    $alias = '/' . $this->randomMachineName();
 
     $language = new Language(array('id' => 'en'));
 
@@ -157,7 +157,7 @@ public function testGetPathByAliasLangcode() {
    * @covers ::getAliasByPath
    */
   public function testGetAliasByPathWhitelist() {
-    $path_part1 = $this->randomMachineName();
+    $path_part1 = '/' . $this->randomMachineName();
     $path_part2 = $this->randomMachineName();
     $path = $path_part1 . '/' . $path_part2;
 
@@ -182,7 +182,7 @@ public function testGetAliasByPathWhitelist() {
    * @covers ::getAliasByPath
    */
   public function testGetAliasByPathNoMatch() {
-    $path_part1 = $this->randomMachineName();
+    $path_part1 = '/' . $this->randomMachineName();
     $path_part2 = $this->randomMachineName();
     $path = $path_part1 . '/' . $path_part2;
 
@@ -219,7 +219,7 @@ public function testGetAliasByPathNoMatch() {
    * @covers ::writeCache
    */
   public function testGetAliasByPathMatch() {
-    $path_part1 = $this->randomMachineName();
+    $path_part1 = '/' . $this->randomMachineName();
     $path_part2 = $this->randomMachineName();
     $path = $path_part1 . '/' . $path_part2;
     $alias = $this->randomMachineName();
@@ -257,7 +257,7 @@ public function testGetAliasByPathMatch() {
    * @covers ::writeCache
    */
   public function testGetAliasByPathCachedMatch() {
-    $path_part1 = $this->randomMachineName();
+    $path_part1 = '/' . $this->randomMachineName();
     $path_part2 = $this->randomMachineName();
     $path = $path_part1 . '/' . $path_part2;
     $alias = $this->randomMachineName();
@@ -304,7 +304,7 @@ public function testGetAliasByPathCachedMatch() {
    * @covers ::writeCache
    */
   public function testGetAliasByPathCachedMissLanguage() {
-    $path_part1 = $this->randomMachineName();
+    $path_part1 = '/' . $this->randomMachineName();
     $path_part2 = $this->randomMachineName();
     $path = $path_part1 . '/' . $path_part2;
     $alias = $this->randomMachineName();
@@ -357,7 +357,7 @@ public function testGetAliasByPathCachedMissLanguage() {
    * @covers ::writeCache
    */
   public function testGetAliasByPathCachedMissNoAlias() {
-    $path_part1 = $this->randomMachineName();
+    $path_part1 = '/' . $this->randomMachineName();
     $path_part2 = $this->randomMachineName();
     $path = $path_part1 . '/' . $path_part2;
     $cached_path = $this->randomMachineName();
@@ -405,7 +405,7 @@ public function testGetAliasByPathCachedMissNoAlias() {
    * @covers ::writeCache
    */
   public function testGetAliasByPathUncachedMissNoAlias() {
-    $path_part1 = $this->randomMachineName();
+    $path_part1 = '/' . $this->randomMachineName();
     $path_part2 = $this->randomMachineName();
     $path = $path_part1 . '/' . $path_part2;
     $cached_path = $this->randomMachineName();
@@ -455,8 +455,8 @@ public function testGetAliasByPathUncachedMissNoAlias() {
    * @covers ::cacheClear
    */
   public function testCacheClear() {
-    $path = 'path';
-    $alias = 'alias';
+    $path = '/path';
+    $alias = '/alias';
     $language = $this->setUpCurrentLanguage();
     $this->aliasStorage->expects($this->exactly(2))
       ->method('lookupPathAlias')
@@ -495,7 +495,7 @@ public function testCacheClear() {
    * @covers ::writeCache
    */
   public function testGetAliasByPathUncachedMissWithAlias() {
-    $path_part1 = $this->randomMachineName();
+    $path_part1 = '/' . $this->randomMachineName();
     $path_part2 = $this->randomMachineName();
     $path = $path_part1 . '/' . $path_part2;
     $cached_path = $this->randomMachineName();
diff --git a/core/tests/Drupal/Tests/Core/Path/PathMatcherTest.php b/core/tests/Drupal/Tests/Core/Path/PathMatcherTest.php
index ad8882e..1d1aaf7 100644
--- a/core/tests/Drupal/Tests/Core/Path/PathMatcherTest.php
+++ b/core/tests/Drupal/Tests/Core/Path/PathMatcherTest.php
@@ -68,43 +68,43 @@ public function getMatchPathData() {
     return array(
       array(
         // Single absolute paths.
-        'example/1',
+        '/example/1',
         array(
-          'example/1' => TRUE,
-          'example/2' => FALSE,
-          'test' => FALSE,
+          '/example/1' => TRUE,
+          '/example/2' => FALSE,
+          '/test' => FALSE,
         ),
       ),
       array(
         // Single paths with wildcards.
-        'example/*',
+        '/example/*',
         array(
-          'example/1' => TRUE,
-          'example/2' => TRUE,
-          'example/3/edit' => TRUE,
-          'example/' => TRUE,
-          'example' => FALSE,
-          'test' => FALSE,
+          '/example/1' => TRUE,
+          '/example/2' => TRUE,
+          '/example/3/edit' => TRUE,
+          '/example/' => TRUE,
+          '/example' => FALSE,
+          '/test' => FALSE,
         ),
       ),
       array(
         // Single paths with multiple wildcards.
-        'node/*/revisions/*',
+        '/node/*/revisions/*',
         array(
-          'node/1/revisions/3' => TRUE,
-          'node/345/revisions/test' => TRUE,
-          'node/23/edit' => FALSE,
-          'test' => FALSE,
+          '/node/1/revisions/3' => TRUE,
+          '/node/345/revisions/test' => TRUE,
+          '/node/23/edit' => FALSE,
+          '/test' => FALSE,
         ),
       ),
       array(
         // Single paths with '<front>'.
         "<front>",
         array(
-          'dummy' => TRUE,
-          "dummy/" => FALSE,
-          "dummy/edit" => FALSE,
-          'node' => FALSE,
+          '/dummy' => TRUE,
+          "/dummy/" => FALSE,
+          "/dummy/edit" => FALSE,
+          '/node' => FALSE,
           '' => FALSE,
         ),
       ),
@@ -112,11 +112,11 @@ public function getMatchPathData() {
         // Paths with both '<front>' and wildcards (should not work).
         "<front>/*",
         array(
-          'dummy' => FALSE,
-          'dummy/' => FALSE,
-          'dummy/edit' => FALSE,
-          'node/12' => FALSE,
-          '' => FALSE,
+          '/dummy' => FALSE,
+          '/dummy/' => FALSE,
+          '/dummy/edit' => FALSE,
+          '/node/12' => FALSE,
+          '/' => FALSE,
         ),
       ),
       array(
diff --git a/core/tests/Drupal/Tests/Core/Routing/UrlGeneratorTest.php b/core/tests/Drupal/Tests/Core/Routing/UrlGeneratorTest.php
index f4e90de..99c00cc 100644
--- a/core/tests/Drupal/Tests/Core/Routing/UrlGeneratorTest.php
+++ b/core/tests/Drupal/Tests/Core/Routing/UrlGeneratorTest.php
@@ -159,12 +159,12 @@ protected function setUp() {
   public function aliasManagerCallback() {
     $args = func_get_args();
     switch($args[0]) {
-      case 'test/one':
-        return 'hello/world';
-      case 'test/two/5':
-        return 'goodbye/cruel/world';
-      case '<front>':
-        return '';
+      case '/test/one':
+        return '/hello/world';
+      case '/test/two/5':
+        return '/goodbye/cruel/world';
+      case '/<front>':
+        return '/';
       default:
         return $args[0];
     }