diff --git a/core/includes/common.inc b/core/includes/common.inc
index 11f8a9d..6ea9bf9 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -9,30 +9,22 @@
  */
 
 use Drupal\Component\Serialization\Json;
-use Drupal\Component\Serialization\Yaml;
-use Drupal\Component\Serialization\Exception\InvalidDataTypeException;
 use Drupal\Component\Utility\Bytes;
 use Drupal\Component\Utility\Crypt;
 use Drupal\Component\Utility\Html;
-use Drupal\Component\Utility\Number;
 use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Component\Utility\SortArray;
 use Drupal\Component\Utility\String;
-use Drupal\Component\Utility\Tags;
 use Drupal\Component\Utility\UrlHelper;
 use Drupal\Core\Cache\Cache;
-use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\PhpStorage\PhpStorageFactory;
 use Drupal\Core\Site\Settings;
 use Drupal\Core\Url;
 use Symfony\Component\HttpFoundation\Response;
 use Symfony\Component\HttpFoundation\Request;
-use Drupal\Core\PhpStorage\PhpStorageFactory;
 use Drupal\Component\Utility\NestedArray;
-use Drupal\Core\Datetime\DrupalDateTime;
-use Drupal\Core\Routing\GeneratorNotInitializedException;
 use Drupal\Core\Template\Attribute;
 use Drupal\Core\Render\Element;
-use Drupal\Core\Session\AnonymousUserSession;
 
 /**
  * @defgroup php_wrappers PHP wrapper functions
diff --git a/core/lib/Drupal/Component/Serialization/Yaml.php b/core/lib/Drupal/Component/Serialization/Yaml.php
index 7d0f0c5..fcc4baf 100644
--- a/core/lib/Drupal/Component/Serialization/Yaml.php
+++ b/core/lib/Drupal/Component/Serialization/Yaml.php
@@ -7,36 +7,56 @@
 
 namespace Drupal\Component\Serialization;
 
-use Drupal\Component\Serialization\Exception\InvalidDataTypeException;
-use Symfony\Component\Yaml\Yaml as Symfony;
-
 /**
  * Default serialization for YAML using the Symfony component.
  */
 class Yaml implements SerializationInterface {
 
   /**
+   * The YAML implementation to use.
+   *
+   * @var \Drupal\Component\Serialization\SerializationInterface
+   */
+  protected static $instance;
+
+  /**
+   * Determines the optimal implementation to use for encoding and parsing Yaml.
+   *
+   * The selection is made based on the enabled PHP extensions, with the most
+   * performant available option chosen.
+   */
+  public function __construct() {
+    if (isset(static::$instance)) {
+      return;
+    }
+    // If the PECL YAML extension installed, use that.
+    if (extension_loaded('yaml')) {
+      static::$instance = new YamlPecl();
+    }
+    else {
+      // Otherwise, fallback to the Symfony implementation.
+      static::$instance = new YamlSymfony();
+    }
+  }
+
+  /**
    * {@inheritdoc}
    */
   public static function encode($data) {
-    try {
-      return Symfony::dump($data, PHP_INT_MAX, 2, TRUE);
-    }
-    catch (\Exception $e) {
-      throw new InvalidDataTypeException($e->getMessage(), $e->getCode(), $e);
+    if (!isset(static::$instance)) {
+      new static();
     }
+    return static::$instance->encode($data);
   }
 
   /**
    * {@inheritdoc}
    */
   public static function decode($raw) {
-    try {
-      return Symfony::parse($raw, TRUE);
-    }
-    catch (\Exception $e) {
-      throw new InvalidDataTypeException($e->getMessage(), $e->getCode(), $e);
+    if (!isset(static::$instance)) {
+      new static();
     }
+    return static::$instance->decode($raw);
   }
 
   /**
diff --git a/core/lib/Drupal/Component/Serialization/YamlPecl.php b/core/lib/Drupal/Component/Serialization/YamlPecl.php
new file mode 100644
index 0000000..7d5b586
--- /dev/null
+++ b/core/lib/Drupal/Component/Serialization/YamlPecl.php
@@ -0,0 +1,113 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Component\Serialization\YamlPecl.
+ */
+
+namespace Drupal\Component\Serialization;
+
+use Drupal\Component\Serialization\Exception\InvalidDataTypeException;
+
+/**
+ * Default serialization for YAML using the PECL extension.
+ */
+class YamlPecl implements SerializationInterface {
+
+  /**
+   * Sets defaults according to Drupal coding standards.
+   */
+  public function __construct() {
+    ini_set('yaml.output_indent', 2);
+    // Do not break lines at 80 characters.
+    ini_set('yaml.output_width', -1);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function encode($data) {
+    return yaml_emit($data, YAML_UTF8_ENCODING, YAML_LN_BREAK);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function decode($raw) {
+    // @todo Use ErrorExceptions.
+    // @see https://drupal.org/node/1247666
+    set_error_handler(array(__CLASS__, 'errorHandler'));
+    $ndocs = 0;
+    $data = yaml_parse($raw, 0, $ndocs, [
+      YAML_BOOL_TAG => '\Drupal\Component\Serialization\YamlPecl::applyBooleanCallbacks',
+      YAML_SEQ_TAG => '\Drupal\Component\Serialization\YamlPecl::applySequenceCallbacks'
+    ]);
+    restore_error_handler();
+    return $data;
+  }
+
+  /**
+   * Temporary error handler for YamlPecl::decode().
+   */
+  public static function errorHandler($severity, $message, $file, $line) {
+    restore_error_handler();
+    throw new InvalidDataTypeException($message, $severity);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getFileExtension() {
+    return 'yml';
+  }
+
+  /**
+   * Applies callbacks after parsing to ignore 1.1 style booleans.
+   *
+   * @param mixed $value
+   *   Value from YAML file.
+   * @param string $tag
+   *   Tag that triggered the callback.
+   * @param int $flags
+   *   Scalar entity style flags.
+   *
+   * @return string|bool
+   *   FALSE, false, TRUE and true are returned as booleans, everything else is
+   *   returned as a string.
+   */
+  public static function applyBooleanCallbacks($value, $tag, $flags) {
+    // YAML 1.1 spec dictates that 'Y', 'N', 'y' and 'n' are booleans, we want
+    // the 1.2 behaviour so we only case 'false', 'FALSE', 'true' OR 'TRUE' as
+    // booleans.
+    if (!in_array(strtolower($value), ['false', 'true'], TRUE)) {
+      return $value;
+    }
+    $map = [
+      'false' => FALSE,
+      'true' => TRUE,
+    ];
+    return $map[strtolower($value)];
+  }
+
+  /**
+   * Applies sequence after parsing to ignore 1.1 issues.
+   *
+   * @param mixed $value
+   *   Value from YAML file.
+   * @param string $tag
+   *   Tag that triggered the callback.
+   * @param int $flags
+   *   Scalar entity style flags.
+   *
+   * @return array|string
+   *   Yaml 1.1 decodes - - as an array containg a NULL, in 1.2 it should be the
+   *   string '-'.
+   */
+  public static function applySequenceCallbacks($value, $tag, $flags) {
+    if ($value == [NULL]) {
+      return '-';
+    }
+    return $value;
+  }
+
+}
diff --git a/core/lib/Drupal/Component/Serialization/Yaml.php b/core/lib/Drupal/Component/Serialization/YamlSymfony.php
similarity index 88%
copy from core/lib/Drupal/Component/Serialization/Yaml.php
copy to core/lib/Drupal/Component/Serialization/YamlSymfony.php
index 7d0f0c5..2d71b7c 100644
--- a/core/lib/Drupal/Component/Serialization/Yaml.php
+++ b/core/lib/Drupal/Component/Serialization/YamlSymfony.php
@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Contains \Drupal\Component\Serialization\Yaml.
+ * Contains \Drupal\Component\Serialization\YamlSymfony.
  */
 
 namespace Drupal\Component\Serialization;
@@ -13,7 +13,7 @@
 /**
  * Default serialization for YAML using the Symfony component.
  */
-class Yaml implements SerializationInterface {
+class YamlSymfony implements SerializationInterface {
 
   /**
    * {@inheritdoc}
diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index 5ff88b2..2be5420 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -12,7 +12,7 @@
 use Drupal\Core\Asset\Exception\LibraryDefinitionMissingLicenseException;
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Component\Serialization\Exception\InvalidDataTypeException;
-use Drupal\Component\Serialization\Yaml;
+use Drupal\Core\Serialization\Yaml;
 
 /**
  * Parses library files to get extension data.
diff --git a/core/lib/Drupal/Core/Config/ConfigManager.php b/core/lib/Drupal/Core/Config/ConfigManager.php
index a97cf72..d83a4ec 100644
--- a/core/lib/Drupal/Core/Config/ConfigManager.php
+++ b/core/lib/Drupal/Core/Config/ConfigManager.php
@@ -8,7 +8,7 @@
 namespace Drupal\Core\Config;
 
 use Drupal\Component\Diff\Diff;
-use Drupal\Component\Serialization\Yaml;
+use Drupal\Core\Serialization\Yaml;
 use Drupal\Core\Config\Entity\ConfigDependencyManager;
 use Drupal\Core\Config\Entity\ConfigEntityDependency;
 use Drupal\Core\Entity\EntityManagerInterface;
diff --git a/core/lib/Drupal/Core/Config/FileStorage.php b/core/lib/Drupal/Core/Config/FileStorage.php
index f404480..384e669 100644
--- a/core/lib/Drupal/Core/Config/FileStorage.php
+++ b/core/lib/Drupal/Core/Config/FileStorage.php
@@ -7,7 +7,7 @@
 
 namespace Drupal\Core\Config;
 
-use Drupal\Component\Serialization\Yaml;
+use Drupal\Core\Serialization\Yaml;
 use Drupal\Component\Serialization\Exception\InvalidDataTypeException;
 use Drupal\Component\Utility\String;
 
diff --git a/core/lib/Drupal/Core/DependencyInjection/YamlFileLoader.php b/core/lib/Drupal/Core/DependencyInjection/YamlFileLoader.php
index d4ae53e..2c2eaef 100644
--- a/core/lib/Drupal/Core/DependencyInjection/YamlFileLoader.php
+++ b/core/lib/Drupal/Core/DependencyInjection/YamlFileLoader.php
@@ -7,7 +7,7 @@
 
 namespace Drupal\Core\DependencyInjection;
 
-use Drupal\Component\Serialization\Yaml;
+use Drupal\Core\Serialization\Yaml;
 use Symfony\Component\DependencyInjection\Alias;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 use Symfony\Component\DependencyInjection\Definition;
diff --git a/core/lib/Drupal/Core/Discovery/YamlDiscovery.php b/core/lib/Drupal/Core/Discovery/YamlDiscovery.php
new file mode 100644
index 0000000..bac6219
--- /dev/null
+++ b/core/lib/Drupal/Core/Discovery/YamlDiscovery.php
@@ -0,0 +1,30 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Discovery\YamlDiscovery.
+ */
+
+namespace Drupal\Core\Discovery;
+
+use Drupal\Core\Serialization\Yaml;
+use Drupal\Component\Discovery\YamlDiscovery as ComponentYamlDiscovery;
+
+/**
+ * Provides discovery for YAML files within a given set of directories.
+ */
+class YamlDiscovery extends ComponentYamlDiscovery {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function findAll() {
+    $all = array();
+    foreach ($this->findFiles() as $provider => $file) {
+      $all[$provider] = Yaml::decode(file_get_contents($file));
+    }
+
+    return $all;
+  }
+
+}
diff --git a/core/lib/Drupal/Core/Extension/InfoParser.php b/core/lib/Drupal/Core/Extension/InfoParser.php
index de1b24a..013453d 100644
--- a/core/lib/Drupal/Core/Extension/InfoParser.php
+++ b/core/lib/Drupal/Core/Extension/InfoParser.php
@@ -7,7 +7,7 @@
 
 namespace Drupal\Core\Extension;
 
-use Drupal\Component\Serialization\Yaml;
+use Drupal\Core\Serialization\Yaml;
 use Drupal\Component\Serialization\Exception\InvalidDataTypeException;
 use Drupal\Component\Utility\String;
 
diff --git a/core/lib/Drupal/Core/Extension/ModuleInstaller.php b/core/lib/Drupal/Core/Extension/ModuleInstaller.php
index 73278d3..48009a1 100644
--- a/core/lib/Drupal/Core/Extension/ModuleInstaller.php
+++ b/core/lib/Drupal/Core/Extension/ModuleInstaller.php
@@ -7,7 +7,7 @@
 
 namespace Drupal\Core\Extension;
 
-use Drupal\Component\Serialization\Yaml;
+use Drupal\Core\Serialization\Yaml;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\DrupalKernelInterface;
diff --git a/core/lib/Drupal/Core/Plugin/Discovery/YamlDiscovery.php b/core/lib/Drupal/Core/Plugin/Discovery/YamlDiscovery.php
index 0b7d10f6..9803171 100644
--- a/core/lib/Drupal/Core/Plugin/Discovery/YamlDiscovery.php
+++ b/core/lib/Drupal/Core/Plugin/Discovery/YamlDiscovery.php
@@ -8,7 +8,7 @@
 namespace Drupal\Core\Plugin\Discovery;
 
 use Drupal\Component\Plugin\Discovery\DiscoveryInterface;
-use Drupal\Component\Discovery\YamlDiscovery as ComponentYamlDiscovery;
+use Drupal\Core\Discovery\YamlDiscovery as CoreYamlDiscovery;
 use Drupal\Component\Plugin\Discovery\DiscoveryTrait;
 
 /**
@@ -35,7 +35,7 @@ class YamlDiscovery implements DiscoveryInterface {
    *   An array of directories to scan.
    */
   function __construct($name, array $directories) {
-    $this->discovery = new ComponentYamlDiscovery($name, $directories);
+    $this->discovery = new CoreYamlDiscovery($name, $directories);
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Serialization/Yaml.php b/core/lib/Drupal/Core/Serialization/Yaml.php
new file mode 100644
index 0000000..ff663d8
--- /dev/null
+++ b/core/lib/Drupal/Core/Serialization/Yaml.php
@@ -0,0 +1,48 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Serialization\Yaml.
+ */
+
+namespace Drupal\Core\Serialization;
+
+use Drupal\Component\Serialization\YamlPecl;
+use Drupal\Component\Serialization\YamlSymfony;
+use Drupal\Core\Site\Settings;
+use Drupal\Component\Serialization\Yaml as ComponentYaml;
+
+/**
+ * Default serialization for YAML.
+ *
+ * Automatically uses the optimal YAML implementation, unless overridden in
+ * Settings.
+ */
+class Yaml extends ComponentYaml {
+
+  /**
+   * Determines the optimal implementation to use for encoding and parsing Yaml.
+   *
+   * The selection is made based on the enabled PHP extensions, with the most
+   * performant available option chosen.
+   */
+  public function __construct() {
+    if (isset(static::$instance)) {
+      return;
+    }
+    $settings = Settings::getInstance();
+    // If there is a settings.php override, use that.
+    if ($settings && ($class = $settings->get('yaml_parser_class'))) {
+      static::$instance = new $class();
+    }
+    // If the PECL YAML extension installed, use that.
+    elseif (extension_loaded('yaml')) {
+      static::$instance = new YamlPecl();
+    }
+    else {
+      // Otherwise, fallback to the Symfony implementation.
+      static::$instance = new YamlSymfony();
+    }
+  }
+
+}
diff --git a/core/modules/migrate_drupal/config/install/migrate.migration.d6_block.yml b/core/modules/migrate_drupal/config/install/migrate.migration.d6_block.yml
index 494752c..28e1821 100644
--- a/core/modules/migrate_drupal/config/install/migrate.migration.d6_block.yml
+++ b/core/modules/migrate_drupal/config/install/migrate.migration.d6_block.yml
@@ -61,7 +61,7 @@ process:
     source:
       - region
       - theme
-      - @theme
+      - '@theme'
     region_map:
       left: sidebar_first
       right: sidebar_second
@@ -75,7 +75,7 @@ process:
   settings:
     plugin: d6_block_settings
     source:
-      - @plugin
+      - '@plugin'
       - delta
       - settings
   visibility:
diff --git a/core/modules/migrate_drupal/config/install/migrate.migration.d6_field.yml b/core/modules/migrate_drupal/config/install/migrate.migration.d6_field.yml
index 72ae39b..0e4b579 100644
--- a/core/modules/migrate_drupal/config/install/migrate.migration.d6_field.yml
+++ b/core/modules/migrate_drupal/config/install/migrate.migration.d6_field.yml
@@ -127,7 +127,7 @@ process:
   settings:
     plugin: field_settings
     source:
-      - @type
+      - '@type'
       - global_settings
       - widget_settings
 
diff --git a/core/modules/migrate_drupal/config/install/migrate.migration.d6_filter_format.yml b/core/modules/migrate_drupal/config/install/migrate.migration.d6_filter_format.yml
index e6c86df..c2123d9 100644
--- a/core/modules/migrate_drupal/config/install/migrate.migration.d6_filter_format.yml
+++ b/core/modules/migrate_drupal/config/install/migrate.migration.d6_filter_format.yml
@@ -19,7 +19,7 @@ process:
   filters:
     plugin: iterator
     source: filters
-    key: @id
+    key: '@id'
     process:
       id:
         plugin: static_map
diff --git a/core/modules/migrate_drupal/config/install/migrate.migration.d6_menu_links.yml b/core/modules/migrate_drupal/config/install/migrate.migration.d6_menu_links.yml
index 1cafbe9..f8fe199 100644
--- a/core/modules/migrate_drupal/config/install/migrate.migration.d6_menu_links.yml
+++ b/core/modules/migrate_drupal/config/install/migrate.migration.d6_menu_links.yml
@@ -27,10 +27,10 @@ process:
     source:
       - link_path
       - options
-  route_name: @route/route_name
-  route_parameters: @route/route_parameters
-  url: @route/url
-  options: @route/options
+  route_name: '@route/route_name'
+  route_parameters: '@route/route_parameters'
+  url: '@route/url'
+  options: '@route/options'
   external: external
   weight: weight
   expanded: expanded
diff --git a/core/modules/migrate_drupal/config/install/migrate.migration.d6_upload_entity_display.yml b/core/modules/migrate_drupal/config/install/migrate.migration.d6_upload_entity_display.yml
index 9e1d7e7..9df3613 100644
--- a/core/modules/migrate_drupal/config/install/migrate.migration.d6_upload_entity_display.yml
+++ b/core/modules/migrate_drupal/config/install/migrate.migration.d6_upload_entity_display.yml
@@ -19,7 +19,7 @@ process:
   field_name: 'constants/name'
   type: 'constants/type'
   options: 'constants/options'
-  'options/type': @type
+  'options/type': '@type'
 destination:
   plugin: component_entity_display
 migration_dependencies:
diff --git a/core/modules/migrate_drupal/config/install/migrate.migration.d6_upload_entity_form_display.yml b/core/modules/migrate_drupal/config/install/migrate.migration.d6_upload_entity_form_display.yml
index f19d033..eb128d5 100644
--- a/core/modules/migrate_drupal/config/install/migrate.migration.d6_upload_entity_form_display.yml
+++ b/core/modules/migrate_drupal/config/install/migrate.migration.d6_upload_entity_form_display.yml
@@ -20,7 +20,7 @@ process:
   form_mode: 'constants/form_mode'
   type: 'constants/type'
   options: 'constants/options'
-  'options/type': @type
+  'options/type': '@type'
 destination:
   plugin: component_entity_form_display
 migration_dependencies:
diff --git a/core/modules/migrate_drupal/config/install/migrate.migration.d6_user_picture_entity_display.yml b/core/modules/migrate_drupal/config/install/migrate.migration.d6_user_picture_entity_display.yml
index afbf438..b53017f 100644
--- a/core/modules/migrate_drupal/config/install/migrate.migration.d6_user_picture_entity_display.yml
+++ b/core/modules/migrate_drupal/config/install/migrate.migration.d6_user_picture_entity_display.yml
@@ -22,7 +22,7 @@ process:
   field_name: 'constants/name'
   type: 'constants/type'
   options: 'constants/options'
-  'options/type': @type
+  'options/type': '@type'
 destination:
   plugin: component_entity_display
 migration_dependencies:
diff --git a/core/modules/migrate_drupal/config/install/migrate.migration.d6_user_picture_entity_form_display.yml b/core/modules/migrate_drupal/config/install/migrate.migration.d6_user_picture_entity_form_display.yml
index d2d7b14..69ca02d 100644
--- a/core/modules/migrate_drupal/config/install/migrate.migration.d6_user_picture_entity_form_display.yml
+++ b/core/modules/migrate_drupal/config/install/migrate.migration.d6_user_picture_entity_form_display.yml
@@ -21,7 +21,7 @@ process:
   form_mode: 'constants/form_mode'
   type: 'constants/type'
   options: 'constants/options'
-  'options/type': @type
+  'options/type': '@type'
 destination:
   plugin: component_entity_form_display
 migration_dependencies:
diff --git a/core/modules/migrate_drupal/config/install/migrate.migration.d6_user_profile_entity_display.yml b/core/modules/migrate_drupal/config/install/migrate.migration.d6_user_profile_entity_display.yml
index 32dcf1b..62ca89e 100644
--- a/core/modules/migrate_drupal/config/install/migrate.migration.d6_user_profile_entity_display.yml
+++ b/core/modules/migrate_drupal/config/install/migrate.migration.d6_user_profile_entity_display.yml
@@ -28,7 +28,7 @@ process:
       textarea: text_default
       url: link_default
   options: 'constants/options'
-  'options/type': @type
+  'options/type': '@type'
   hidden:
     plugin: static_map
     source: visibility
diff --git a/core/modules/migrate_drupal/config/install/migrate.migration.d6_user_profile_entity_form_display.yml b/core/modules/migrate_drupal/config/install/migrate.migration.d6_user_profile_entity_form_display.yml
index e09b608..4b33943 100644
--- a/core/modules/migrate_drupal/config/install/migrate.migration.d6_user_profile_entity_form_display.yml
+++ b/core/modules/migrate_drupal/config/install/migrate.migration.d6_user_profile_entity_form_display.yml
@@ -26,11 +26,11 @@ process:
       textarea: text_textarea
       url: link_default
   options: 'constants/options'
-  'options/type': @type
+  'options/type': '@type'
   'options/settings':
     plugin: field_instance_widget_settings
     source:
-      - @type
+      - '@type'
       - 'constants/empty' # we don't have any settings.
   'options/settings/display_label':  # Single on/off checkboxes need to have display_label = true otherwise their label doesn't show.
     plugin: static_map
diff --git a/core/modules/migrate_drupal/config/install/migrate.migration.d6_vocabulary_field.yml b/core/modules/migrate_drupal/config/install/migrate.migration.d6_vocabulary_field.yml
index a833cbd..a1392e2 100644
--- a/core/modules/migrate_drupal/config/install/migrate.migration.d6_vocabulary_field.yml
+++ b/core/modules/migrate_drupal/config/install/migrate.migration.d6_vocabulary_field.yml
@@ -16,7 +16,7 @@ process:
     plugin: migration
     migration: d6_taxonomy_vocabulary
     source: vid
-  'settings/allowed_values/0/vocabulary': @field_name
+  'settings/allowed_values/0/vocabulary': '@field_name'
   'settings/allowed_values/0/parent': 'constants/parent'
   cardinality: 'constants/cardinality'
 destination:
diff --git a/core/modules/system/src/Tests/Extension/InfoParserUnitTest.php b/core/modules/system/src/Tests/Extension/InfoParserUnitTest.php
index b925285..4669e03 100644
--- a/core/modules/system/src/Tests/Extension/InfoParserUnitTest.php
+++ b/core/modules/system/src/Tests/Extension/InfoParserUnitTest.php
@@ -81,7 +81,8 @@ public function testInfoParser() {
     $info_values = $this->infoParser->parse('core/modules/system/tests/fixtures/common_test.info.txt');
     $this->assertEqual($info_values['simple_string'], 'A simple string', 'Simple string value was parsed correctly.', 'System');
     $this->assertEqual($info_values['version'], \Drupal::VERSION, 'Constant value was parsed correctly.', 'System');
-    $this->assertEqual($info_values['double_colon'], 'dummyClassName::', 'Value containing double-colon was parsed correctly.', 'System');
+    debug($info_values);
+    $this->assertEqual($info_values['double_colon'], 'dummyClassName::foo', 'Value containing double-colon was parsed correctly.', 'System');
   }
 
 }
diff --git a/core/modules/system/tests/fixtures/common_test.info.txt b/core/modules/system/tests/fixtures/common_test.info.txt
index 7e57dfe..ff535b1 100644
--- a/core/modules/system/tests/fixtures/common_test.info.txt
+++ b/core/modules/system/tests/fixtures/common_test.info.txt
@@ -4,4 +4,4 @@ type: module
 description: 'testing info file parsing'
 simple_string: 'A simple string'
 version: "VERSION"
-double_colon: dummyClassName::
+double_colon: dummyClassName::foo
diff --git a/core/modules/text/config/schema/text.data_types.schema.yml b/core/modules/text/config/schema/text.data_types.schema.yml
deleted file mode 100644
index e69de29..0000000
diff --git a/core/modules/user/config/schema/user.views.schema.yml b/core/modules/user/config/schema/user.views.schema.yml
index 31b3950..6089c6f 100644
--- a/core/modules/user/config/schema/user.views.schema.yml
+++ b/core/modules/user/config/schema/user.views.schema.yml
@@ -68,17 +68,13 @@ views_field_user:
       label: 'Link this field to its user'
 
 views.field.user_language:
-  type: views_field
+  type: views_field_user
   label: 'User language'
   mapping:
     link_to_node:
       type: boolean
       label: 'Link this field to its user'
 
-views.field.user_language:
-  type: views_field_user
-  label: 'User language'
-
 views.field.user_link:
   type: views_field
   label: 'User link'
diff --git a/core/modules/views/config/schema/views.data_types.schema.yml b/core/modules/views/config/schema/views.data_types.schema.yml
index 64ff9c4..7cf0cad 100644
--- a/core/modules/views/config/schema/views.data_types.schema.yml
+++ b/core/modules/views/config/schema/views.data_types.schema.yml
@@ -365,7 +365,6 @@ views_argument:
       label: 'Override title'
     title:
       type: label
-      label: 'Title'
       label: 'Overridden title'
     default_argument_type:
       type: string
diff --git a/core/tests/Drupal/Tests/Component/Serialization/YamlTest.php b/core/tests/Drupal/Tests/Component/Serialization/YamlTest.php
new file mode 100644
index 0000000..2bd3764
--- /dev/null
+++ b/core/tests/Drupal/Tests/Component/Serialization/YamlTest.php
@@ -0,0 +1,157 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Component\Serialization\YamlTest.
+ */
+
+namespace Drupal\Tests\Component\Serialization;
+
+use Drupal\Component\Serialization\YamlSymfony;
+use Drupal\Component\Serialization\YamlPecl;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * Tests the \Drupal\Component\Serialization\Yaml* implementations.
+ *
+ * @group Drupal
+ * @group Serialization
+ */
+class YamlTest extends UnitTestCase {
+
+  protected function setUp() {
+    if (!extension_loaded('yaml')) {
+      $this->markTestSkipped('The PECL Yaml extension is not available.');
+    }
+  }
+
+  /**
+   * Tests encoding with Symfony and decoding with PECL and vice versa.
+   *
+   * @dataProvider providerYamlData
+   * @covers \Drupal\Component\Serialization\Yaml::encode
+   * @covers \Drupal\Component\Serialization\Yaml::decode
+   * @covers \Drupal\Component\Serialization\YamlPecl::encode
+   * @covers \Drupal\Component\Serialization\YamlPecl::decode
+   * @covers \Drupal\Component\Serialization\YamlSymfony::encode
+   * @covers \Drupal\Component\Serialization\YamlSymfony::decode
+   */
+  public function testEncodeDecode($data, $parser_class, $dumper_class) {
+    $dumper = new $dumper_class;
+    $parser = new $parser_class;
+
+    $dumped = $dumper->encode($data);
+
+    $parsed = $parser->decode($dumped);
+
+    $this->assertEquals($data, $parsed);
+  }
+
+  /**
+   * Tests decoding YAML node anchors with both Symfony and PECL.
+   *
+   * @dataProvider providerYamlNodeAnchors
+   */
+  public function testDecodeNodeAnchors($data) {
+    $symfony = new YamlSymfony();
+    $pecl = new YamlPecl();
+    $this->assertEquals($symfony->decode($data), $pecl->decode($data));
+  }
+
+  /**
+   * Data provider for YAML instance tests.
+   *
+   * @return array
+   */
+  public function providerYamlData() {
+    $object = array(
+      'foo' => 'bar',
+      'id' => 'schnitzel',
+      'ponies' => array('nope', 'thanks'),
+      'how' => array(
+        'about' => 'if',
+        'i' => 'ask',
+        'nicely'
+      ),
+      'the' => array(
+        'answer' => array(
+          'still' => 'would',
+          'be' => 'Y',
+        ),
+      ),
+      'how_many_times' => 123,
+      'should_i_ask' => FALSE,
+      1,
+      FALSE,
+      array(1, FALSE),
+      array(10),
+      array(0 => '123456'),
+    );
+
+    // Test parsing with Symfony and dumping with PECL.
+    $data[] = array(
+      $object,
+      'Drupal\Component\Serialization\YamlSymfony',
+      'Drupal\Component\Serialization\YamlPecl'
+    );
+    // Test parsing with PECL and dumping with Symfony.
+    $data[] = array(
+      $object,
+      'Drupal\Component\Serialization\YamlPecl',
+      'Drupal\Component\Serialization\YamlSymfony'
+    );
+    return $data;
+  }
+
+  /**
+   * Data provider for YAML instance tests.
+   *
+   * @return array
+   */
+  public function providerYamlNodeAnchors() {
+    $yaml = <<<EOF
+jquery.ui:
+  version: &jquery_ui 1.10.2
+
+jquery.ui.accordion:
+  version: *jquery_ui
+EOF;
+    $data[] = array($yaml);
+    return $data;
+  }
+
+  /**
+   * Tests all YAML files are decoded in the same way with both Symfony and PECL.
+   *
+   * @dataProvider providerYamlFilesInCore
+   */
+  public function testYamlFilesInCore($file) {
+    $data = file_get_contents($file);
+    $symfony = new YamlSymfony();
+    $pecl = new YamlPecl();
+    $this->assertEquals($symfony->decode($data), $pecl->decode($data));
+  }
+
+  /**
+   * Data provider for YAML files in core test.
+   * @return array
+   */
+  public function providerYamlFilesInCore() {
+    $files = array();
+    $dirs = new \RecursiveIteratorIterator(new \RecursiveDirectoryIterator(__DIR__ . '/../../../../../', \RecursiveDirectoryIterator::FOLLOW_SYMLINKS));
+    foreach ($dirs as $dir) {
+      $pathname = $dir->getPathname();
+      // Exclude vendor.
+      if ($dir->getExtension() == 'yml' && strpos($pathname, '/../../../../../vendor') === FALSE) {
+        if (strpos($dir->getRealPath(), 'invalid_file') !== FALSE) {
+          // There are some intentionally invalid files provided for testing
+          // library API behaviours, ignore them.
+          continue;
+        }
+        $files[] = array($dir->getRealPath());
+      }
+    }
+    return $files;
+  }
+
+}