diff --git a/core/lib/Drupal/Core/Condition/ConditionInterface.php b/core/lib/Drupal/Core/Condition/ConditionInterface.php
index fd1b374..bb3d5b8 100644
--- a/core/lib/Drupal/Core/Condition/ConditionInterface.php
+++ b/core/lib/Drupal/Core/Condition/ConditionInterface.php
@@ -54,6 +54,10 @@
    *
    * @return bool
    *   Whether the condition result will be negated.
+   *
+   * @deprecated Deprecated as of 8.0.0. Schedule for removal before Drupal
+   *   9.0.0. Condition negation is an implementation detail and should not be
+   *   relied upon.
    */
   public function isNegated();
 
diff --git a/core/lib/Drupal/Core/Condition/ConditionManager.php b/core/lib/Drupal/Core/Condition/ConditionManager.php
index 0fef97b..890ef5f 100644
--- a/core/lib/Drupal/Core/Condition/ConditionManager.php
+++ b/core/lib/Drupal/Core/Condition/ConditionManager.php
@@ -68,8 +68,11 @@ public function createInstance($plugin_id, array $configuration = array()) {
    * Implements Drupal\Core\Executable\ExecutableManagerInterface::execute().
    */
   public function execute(ExecutableInterface $condition) {
-    $result = $condition->evaluate();
-    return $condition->isNegated() ? !$result : $result;
+    if (!($condition instanceof ConditionInterface)) {
+      throw new \InvalidArgumentException(sprintf('%s must be an instance of %s, but %s was given.', '$condition', '\Drupal\Core\Condition\ConditionInterface', get_class($condition)));
+    }
+
+    return $condition->evaluate();
   }
 
 }
diff --git a/core/modules/language/src/Plugin/Condition/Language.php b/core/modules/language/src/Plugin/Condition/Language.php
index 77fdc08..f87d87a 100644
--- a/core/modules/language/src/Plugin/Condition/Language.php
+++ b/core/modules/language/src/Plugin/Condition/Language.php
@@ -143,7 +143,8 @@ public function evaluate() {
 
     $language = $this->getContextValue('language');
     // Language visibility settings.
-    return !empty($this->configuration['langcodes'][$language->getId()]);
+    $result = !empty($this->configuration['langcodes'][$language->getId()]);
+    return $this->configuration['negate'] ? !$result : $result;
   }
 
   /**
diff --git a/core/modules/system/src/Plugin/Condition/CurrentThemeCondition.php b/core/modules/system/src/Plugin/Condition/CurrentThemeCondition.php
index 3b82d48..46eb74c 100644
--- a/core/modules/system/src/Plugin/Condition/CurrentThemeCondition.php
+++ b/core/modules/system/src/Plugin/Condition/CurrentThemeCondition.php
@@ -109,7 +109,9 @@ public function evaluate() {
       return TRUE;
     }
 
-    return $this->themeManager->getActiveTheme()->getName() == $this->configuration['theme'];
+    $result = $this->themeManager->getActiveTheme()->getName() == $this->configuration['theme'];
+
+    return $this->configuration['negate'] ? !$result : $result;
   }
 
   /**
diff --git a/core/modules/system/src/Plugin/Condition/RequestPath.php b/core/modules/system/src/Plugin/Condition/RequestPath.php
index 64331be..0e1a6d0 100644
--- a/core/modules/system/src/Plugin/Condition/RequestPath.php
+++ b/core/modules/system/src/Plugin/Condition/RequestPath.php
@@ -148,15 +148,18 @@ public function evaluate() {
     // with different case. Ex: /Page, /page, /PAGE.
     $pages = Unicode::strtolower($this->configuration['pages']);
     if (!$pages) {
-      return TRUE;
+      $result = TRUE;
     }
+    else {
+      $request = $this->requestStack->getCurrentRequest();
+      // Compare the lowercase path alias (if any) and internal path.
+      $path = rtrim($this->currentPath->getPath($request), '/');
+      $path_alias = Unicode::strtolower($this->aliasManager->getAliasByPath($path));
 
-    $request = $this->requestStack->getCurrentRequest();
-    // Compare the lowercase path alias (if any) and internal path.
-    $path = rtrim($this->currentPath->getPath($request), '/');
-    $path_alias = Unicode::strtolower($this->aliasManager->getAliasByPath($path));
+      $result = $this->pathMatcher->matchPath($path_alias, $pages) || (($path != $path_alias) && $this->pathMatcher->matchPath($path, $pages));
+    }
 
-    return $this->pathMatcher->matchPath($path_alias, $pages) || (($path != $path_alias) && $this->pathMatcher->matchPath($path, $pages));
+    return $this->configuration['negate'] ? !$result : $result;
   }
 
   /**
diff --git a/core/tests/Drupal/Tests/Core/Condition/ConditionManagerTest.php b/core/tests/Drupal/Tests/Core/Condition/ConditionManagerTest.php
new file mode 100644
index 0000000..ae400ba
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Condition/ConditionManagerTest.php
@@ -0,0 +1,77 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Core\Condition\ConditionManagerTest.
+ */
+
+namespace Drupal\Tests\Core\Condition;
+
+use Drupal\Core\Condition\ConditionManager;
+use Drupal\Tests\UnitTestCase;
+use Zend\Stdlib\ArrayObject;
+
+/**
+ * @coversDefaultClass \Drupal\Core\Condition\ConditionManager
+ *
+ * @group Condition
+ */
+class ConditionManagerTest extends UnitTestCase {
+
+  /**
+   * The cache backend.
+   *
+   * @var \Drupal\Core\Cache\CacheBackendInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  public $cache;
+
+  /**
+   * The module handler.
+   *
+   * @var \Drupal\Core\Extension\ModuleHandlerInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $moduleHandler;
+
+  /**
+   * The class under test.
+   *
+   * @var \Drupal\Core\Condition\ConditionManager
+   */
+  public $sut;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setUp() {
+    $this->moduleHandler = $this->getMock('\Drupal\Core\Extension\ModuleHandlerInterface');
+
+    $this->cache = $this->getMock('\Drupal\Core\Cache\CacheBackendInterface');
+
+    $namespaces = new ArrayObject();
+
+    $this->sut = new ConditionManager($namespaces, $this->cache, $this->moduleHandler);
+  }
+
+  /**
+   * @covers ::__construct
+   */
+  public function testConstruct() {
+    $namespaces = new ArrayObject();
+    $this->sut = new ConditionManager($namespaces, $this->cache, $this->moduleHandler);
+  }
+
+  /**
+   * @covers ::execute
+   */
+  public function testExecute() {
+    $execution_result = $this->randomMachineName();
+
+    $condition = $this->getMock('\Drupal\Core\Condition\ConditionInterface');
+    $condition->expects($this->atLeastOnce())
+      ->method('evaluate')
+      ->willReturn($execution_result);
+
+    $this->assertSame($execution_result, $this->sut->execute($condition));
+  }
+
+}