diff --git a/core/core.libraries.yml b/core/core.libraries.yml
index a6d963c..7bb80a4 100644
--- a/core/core.libraries.yml
+++ b/core/core.libraries.yml
@@ -14,7 +14,7 @@ backbone:
 
 classList:
   remote: https://github.com/eligrey/classList.js
-  version: 2014-12-13
+  version: "2014-12-13"
   license:
     name: Public Domain
     url: https://github.com/eligrey/classList.js/blob/2014-12-13/LICENSE.md
diff --git a/core/lib/Drupal/Component/Serialization/Yaml.php b/core/lib/Drupal/Component/Serialization/Yaml.php
index 8f75ffd..f7bc09a 100644
--- a/core/lib/Drupal/Component/Serialization/Yaml.php
+++ b/core/lib/Drupal/Component/Serialization/Yaml.php
@@ -25,20 +25,16 @@ class Yaml implements SerializationInterface {
    * {@inheritdoc}
    */
   public static function encode($data) {
-    if (!isset(static::$serializer)) {
-      static::ensureSerializer();
-    }
-    return static::$serializer->encode($data);
+    $serializer = static::getSerializer();
+    return $serializer::encode($data);
   }
 
   /**
    * {@inheritdoc}
    */
   public static function decode($raw) {
-    if (!isset(static::$serializer)) {
-      static::ensureSerializer();
-    }
-    return static::$serializer->decode($raw);
+    $serializer = static::getSerializer();
+    return $serializer::decode($raw);
   }
 
   /**
@@ -51,18 +47,18 @@ public static function getFileExtension() {
   /**
    * Determines the optimal implementation to use for encoding and parsing Yaml.
    */
-  protected static function ensureSerializer() {
+  protected static function getSerializer() {
 
-    if (isset(static::$serializer)) {
-      return;
-    }
-    // If the PECL YAML extension installed, use that.
-    if (extension_loaded('yaml')) {
-      static::$serializer = new YamlPecl();
-    }
-    else {
-      // Otherwise, fallback to the Symfony implementation.
-      static::$serializer = new YamlSymfony();
+    if (!isset(static::$serializer)) {
+      // If the PECL YAML extension installed, use that.
+      if (extension_loaded('yaml')) {
+        static::$serializer = '\Drupal\Component\Serialization\YamlPecl';
+      }
+      else {
+        // Otherwise, fallback to the Symfony implementation.
+        static::$serializer = '\Drupal\Component\Serialization\YamlSymfony';
+      }
     }
+    return static::$serializer;
   }
 }
diff --git a/core/lib/Drupal/Component/Serialization/YamlPecl.php b/core/lib/Drupal/Component/Serialization/YamlPecl.php
index 96ef967..0fd833c 100644
--- a/core/lib/Drupal/Component/Serialization/YamlPecl.php
+++ b/core/lib/Drupal/Component/Serialization/YamlPecl.php
@@ -15,18 +15,16 @@
 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) {
+    static $init;
+    if (!isset($init)) {
+      ini_set('yaml.output_indent', 2);
+      // Do not break lines at 80 characters.
+      ini_set('yaml.output_width', -1);
+      $init = TRUE;
+    }
     return yaml_emit($data, YAML_UTF8_ENCODING, YAML_LN_BREAK);
   }
 
@@ -34,6 +32,10 @@ public static function encode($data) {
    * {@inheritdoc}
    */
   public static function decode($raw) {
+    // yaml_parse() will error with an empty value.
+    if (!trim($raw)) {
+      return NULL;
+    }
     // @todo Use ErrorExceptions when https://drupal.org/node/1247666 is in.
     // yaml_parse() will throw errors instead of raising an exception. Until
     // such time as Drupal supports native PHP ErrorExceptions as the error
@@ -42,7 +44,7 @@ public static function decode($raw) {
     // parsing errors into a throwable exception.
     // @see Drupal\Component\Serialization\Exception\InvalidDataTypeException
     // @see http://php.net/manual/en/class.errorexception.php
-    set_error_handler(array(__CLASS__, 'errorHandler'));
+    set_error_handler([__CLASS__, 'errorHandler']);
     $ndocs = 0;
     $data = yaml_parse($raw, 0, $ndocs, [
       YAML_BOOL_TAG => '\Drupal\Component\Serialization\YamlPecl::applyBooleanCallbacks',
diff --git a/core/lib/Drupal/Core/Serialization/Yaml.php b/core/lib/Drupal/Core/Serialization/Yaml.php
index ca6fb71..6b70775 100644
--- a/core/lib/Drupal/Core/Serialization/Yaml.php
+++ b/core/lib/Drupal/Core/Serialization/Yaml.php
@@ -20,18 +20,14 @@ class Yaml extends ComponentYaml {
   /**
    * {@inheritdoc}
    */
-  protected static function ensureSerializer(){
-    if (isset(static::$serializer)) {
-      return;
-    }
-
+  protected static function getSerializer(){
     // If there is a settings.php override, use that.
-    if ($class = Settings::get('yaml_parser_class')) {
-      static::$serializer = new $class();
-    }
-    else {
-      parent::ensureSerializer();
+    if (!isset(static::$serializer) &&
+      $class = Settings::get('yaml_parser_class')) {
+
+      static::$serializer = $class;
     }
+    return parent::getSerializer();
   }
 
 }
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.services.yml b/core/modules/system/tests/modules/theme_test/theme_test.services.yml
index add8b4c..9049f36 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.services.yml
+++ b/core/modules/system/tests/modules/theme_test/theme_test.services.yml
@@ -1,7 +1,7 @@
 services:
   theme_test.subscriber:
     class: Drupal\theme_test\EventSubscriber\ThemeTestSubscriber
-    arguments: [@current_route_match]
+    arguments: ["@current_route_match"]
     tags:
       - { name: event_subscriber }
 
diff --git a/core/tests/Drupal/Tests/Component/Serialization/YamlBaseTest.php b/core/tests/Drupal/Tests/Component/Serialization/YamlBaseTest.php
new file mode 100644
index 0000000..36d0637
--- /dev/null
+++ b/core/tests/Drupal/Tests/Component/Serialization/YamlBaseTest.php
@@ -0,0 +1,101 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Component\Serialization\YamlPeclTest.
+ */
+
+namespace Drupal\Tests\Component\Serialization;
+
+/**
+ * Provides test cases for Yaml tests so we can assert certain things parse the
+ * same.
+ */
+abstract class YamlBaseTest extends \PHPUnit_Framework_TestCase {
+
+  /**
+   * Some data sets that should be able to be serialized.
+   *
+   * @return array
+   */
+  public function providerEncodeDecodeTests() {
+    return [
+      [
+        'foo' => 'bar',
+        'id' => 'schnitzel',
+        'ponies' => ['nope', 'thanks'],
+        'how' => [
+          'about' => 'if',
+          'i' => 'ask',
+          'nicely'
+        ],
+        'the' => [
+          'answer' => [
+            'still' => 'would',
+            'be' => 'Y',
+          ],
+        ],
+        'how_many_times' => 123,
+        'should_i_ask' => FALSE,
+        1,
+        FALSE,
+        [1, FALSE],
+        [10],
+        [0 => '123456'],
+      ],
+      [NULL]
+    ];
+  }
+
+  public function providerDecodeTests() {
+    $data = [
+      // Null files.
+      ['', NULL],
+      ["\n", NULL],
+      ["---\n...\n", NULL],
+
+      // Node Anchors.
+      [
+        "
+jquery.ui:
+  version: &jquery_ui 1.10.2
+
+jquery.ui.accordion:
+  version: *jquery_ui
+",
+        [
+          'jquery.ui' => [
+            'version' => '1.10.2'
+          ],
+          'jquery.ui.accordion' => [
+            'version' => '1.10.2'
+          ],
+        ],
+      ],
+    ];
+
+    // 1.2 Bool values.
+    foreach ($this->providerBoolTest() as $test) {
+      $data[] = ['bool: ' . $test[0], ['bool' => $test[1]]];
+    }
+    $data = array_merge($data, $this->providerBoolTest());
+
+    return $data;
+  }
+
+  public function providerBoolTest() {
+    return [
+      ['true', TRUE],
+      ['TRUE', TRUE],
+      ['True', TRUE],
+      ['y', 'y'],
+      ['Y', 'Y'],
+      ['false', FALSE],
+      ['FALSE', FALSE],
+      ['False', FALSE],
+      ['n', 'n'],
+      ['N', 'N'],
+    ];
+  }
+
+}
diff --git a/core/tests/Drupal/Tests/Component/Serialization/YamlPeclTest.php b/core/tests/Drupal/Tests/Component/Serialization/YamlPeclTest.php
index 0705e1f..33030f9 100644
--- a/core/tests/Drupal/Tests/Component/Serialization/YamlPeclTest.php
+++ b/core/tests/Drupal/Tests/Component/Serialization/YamlPeclTest.php
@@ -7,151 +7,80 @@
 
 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.
+ * Test the YamlPecl serialization implementation.
  *
  * @group Drupal
  * @group Serialization
+ * @coversDefaultClass \Drupal\Component\Serialization\YamlPecl
+ * @requires extension yaml
  */
-class YamlPeclTest extends UnitTestCase {
-
-  protected function setUp() {
-    if (!extension_loaded('yaml')) {
-      $this->markTestSkipped('The PECL Yaml extension is not available.');
-    }
-  }
+class YamlPeclTest extends YamlBaseTest {
 
   /**
-   * Tests encoding with Symfony and decoding with PECL and vice versa.
+   * Tests encoding and decoding basic data structures.
    *
-   * @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
+   * @covers ::encode
+   * @covers ::decode
+   * @dataProvider providerEncodeDecodeTests
    */
-  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, $parsed);
+  public function testEncodeDecode($data) {
+    $this->assertEquals($data, YamlPecl::decode(YamlPecl::encode($data)));
   }
 
   /**
-   * Tests decoding YAML node anchors with both Symfony and PECL.
+   * Tests decoding YAML node anchors.
    *
-   * @dataProvider providerYamlNodeAnchors
+   * @covers ::decode
+   * @dataProvider providerDecodeTests
    */
-  public function testDecodeNodeAnchors($data) {
-    $symfony = new YamlSymfony();
-    $pecl = new YamlPecl();
-    $this->assertEquals($symfony->decode($data), $pecl->decode($data));
+  public function testDecode($string, $data) {
+    $this->assertEquals($data, YamlPecl::decode($string));
   }
 
   /**
-   * Data provider for YAML instance tests.
+   * Tests our encode settings.
    *
-   * @return array
+   * @covers ::encode
    */
-  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;
+  public function testEncode() {
+    $this->assertEquals('---
+foo:
+  bar: Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vivamus sapien ex, venenatis vitae nisi eu, posuere luctus dolor. Nullam convallis
+...
+', YamlPecl::encode(['foo' => ['bar' => 'Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vivamus sapien ex, venenatis vitae nisi eu, posuere luctus dolor. Nullam convallis']]));
   }
 
   /**
-   * Data provider for YAML instance tests.
+   * Test yaml boolean callback.
    *
-   * @return array
+   * @covers ::applyBooleanCallbacks
+   * @dataProvider providerBoolTest
+   * @param string $string
+   *   String value for the yaml boolean.
+   * @param string|bool $expected
+   *   The expected return
    */
-  public function providerYamlNodeAnchors() {
-    $yaml = <<<EOF
-jquery.ui:
-  version: &jquery_ui 1.10.2
-
-jquery.ui.accordion:
-  version: *jquery_ui
-EOF;
-    $data[] = array($yaml);
-    return $data;
+  public function testApplyBooleanCallbacks($string, $expected) {
+    $this->assertEquals($expected, YamlPecl::applyBooleanCallbacks($string, 'bool', NULL));
   }
 
   /**
-   * Tests all YAML files are decoded in the same way with both Symfony and PECL.
-   *
-   * @dataProvider providerYamlFilesInCore
+   * @covers ::getFileExtension
    */
-  public function testYamlFilesInCore($file) {
-    $data = file_get_contents($file);
-    $symfony = new YamlSymfony();
-    $pecl = new YamlPecl();
-    $this->assertEquals($symfony->decode($data), $pecl->decode($data));
+  public function testGetFileExtension() {
+    $this->assertEquals('yml', YamlPecl::getFileExtension());
   }
 
   /**
-   * Data provider for YAML files in core test.
-   * @return array
+   * Invalid yaml throws exception.
+   *
+   * @covers ::errorHandler
+   * @expectedException \Drupal\Component\Serialization\Exception\InvalidDataTypeException
    */
-  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 $files;
+  public function testError() {
+    YamlPecl::decode('foo: [ads');
   }
-
 }
diff --git a/core/tests/Drupal/Tests/Component/Serialization/YamlSymfonyTest.php b/core/tests/Drupal/Tests/Component/Serialization/YamlSymfonyTest.php
new file mode 100644
index 0000000..0836be4
--- /dev/null
+++ b/core/tests/Drupal/Tests/Component/Serialization/YamlSymfonyTest.php
@@ -0,0 +1,70 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Component\Serialization\YamlSymfonyTest.
+ */
+
+namespace Drupal\Tests\Component\Serialization;
+
+use Drupal\Component\Serialization\YamlSymfony;
+
+/**
+ * Test the YamlSymfony serialization implementation.
+ *
+ * @group Drupal
+ * @group Serialization
+ * @coversDefaultClass \Drupal\Component\Serialization\YamlSymfony
+ * @requires extension yaml
+ */
+class YamlSymfonyTest extends YamlBaseTest {
+
+  /**
+   * Tests encoding and decoding basic data structures.
+   *
+   * @covers ::encode
+   * @covers ::decode
+   * @dataProvider providerEncodeDecodeTests
+   */
+  public function testEncodeDecode($data) {
+    $this->assertEquals($data, YamlSymfony::decode(YamlSymfony::encode($data)));
+  }
+
+  /**
+   * Tests decoding YAML node anchors.
+   *
+   * @covers ::decode
+   * @dataProvider providerDecodeTests
+   */
+  public function testDecode($string, $data) {
+    $this->assertEquals($data, YamlSymfony::decode($string));
+  }
+
+  /**
+   * Tests our encode settings.
+   *
+   * @covers ::encode
+   */
+  public function testEncode() {
+    $this->assertEquals('foo:
+  bar: \'Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vivamus sapien ex, venenatis vitae nisi eu, posuere luctus dolor. Nullam convallis\'
+', YamlSymfony::encode(['foo' => ['bar' => 'Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vivamus sapien ex, venenatis vitae nisi eu, posuere luctus dolor. Nullam convallis']]));
+  }
+
+  /**
+   * @covers ::getFileExtension
+   */
+  public function testGetFileExtension() {
+    $this->assertEquals('yml', YamlSymfony::getFileExtension());
+  }
+
+  /**
+   * Invalid yaml throws exception.
+   *
+   * @covers ::decode
+   * @expectedException \Drupal\Component\Serialization\Exception\InvalidDataTypeException
+   */
+  public function testError() {
+    YamlSymfony::decode('foo: [ads');
+  }
+}
diff --git a/core/tests/Drupal/Tests/Component/Serialization/YamlTest.php b/core/tests/Drupal/Tests/Component/Serialization/YamlTest.php
index 94269e6..b15898f 100644
--- a/core/tests/Drupal/Tests/Component/Serialization/YamlTest.php
+++ b/core/tests/Drupal/Tests/Component/Serialization/YamlTest.php
@@ -7,7 +7,11 @@
 
 namespace Drupal\Tests\Component\Serialization;
 
+use Drupal\Component\Serialization\Exception\InvalidDataTypeException;
+use Drupal\Component\Serialization\SerializationInterface;
 use Drupal\Component\Serialization\Yaml;
+use Drupal\Component\Serialization\YamlPecl;
+use Drupal\Component\Serialization\YamlSymfony;
 use Drupal\Tests\UnitTestCase;
 
 /**
@@ -20,44 +24,117 @@ class YamlTest extends UnitTestCase {
    * @covers ::decode
    */
   public function testDecode() {
-    // Test that files without line break endings are properly interpreted.
-    $yaml = 'foo: bar';
-    $expected = array(
-      'foo' => 'bar',
-    );
-    $this->assertSame($expected, Yaml::decode($yaml));
-    $yaml .= "\n";
-    $this->assertSame($expected, Yaml::decode($yaml));
-    $yaml .= "\n";
-    $this->assertSame($expected, Yaml::decode($yaml));
-
-    $yaml = "{}\n";
-    $expected = array();
-    $this->assertSame($expected, Yaml::decode($yaml));
-
-    $yaml = '';
-    $this->assertNULL(Yaml::decode($yaml));
-    $yaml .= "\n";
-    $this->assertNULL(Yaml::decode($yaml));
-    $yaml .= "\n";
-    $this->assertNULL(Yaml::decode($yaml));
+    $stub = $this->getMockBuilder('\stdClass')
+      ->setMethods(['encode', 'decode', 'getFileExtension'])
+      ->getMock();
+    $stub
+      ->expects($this->once())
+      ->method('decode');
+    YamlParserProxy::setMock($stub);
+    YamlStub::decode('test');
   }
 
   /**
    * @covers ::encode
    */
   public function testEncode() {
-    $decoded = array(
-      'foo' => 'bar',
-    );
-    $this->assertSame('foo: bar' . "\n", Yaml::encode($decoded));
+    $stub = $this->getMockBuilder('\stdClass')
+      ->setMethods(['encode', 'decode', 'getFileExtension'])
+      ->getMock();
+    $stub
+      ->expects($this->once())
+      ->method('encode');
+    YamlParserProxy::setMock($stub);
+    YamlStub::encode([]);
   }
 
   /**
    * @covers ::getFileExtension
    */
   public function testGetFileExtension() {
-    $this->assertEquals('yml', Yaml::getFileExtension());
+    $stub = $this->getMockBuilder('\stdClass')
+      ->setMethods(['encode', 'decode', 'getFileExtension'])
+      ->getMock();
+    $stub
+      ->expects($this->never())
+      ->method('getFileExtension');
+    YamlParserProxy::setMock($stub);
+    $this->assertEquals('yml', YamlStub::getFileExtension());
   }
 
+  /**
+   * Tests all YAML files are decoded in the same way with both Symfony and PECL.
+   *
+   * This tests is a little bit slow but it tests that we don't have any bugs
+   * in our YAML that might not be decoded correctly in on of our
+   * implementations.
+   *
+   * @TODO this should exist as an integration test no as part of our unit tests.
+   *
+   * @requires extension yaml
+   * @dataProvider providerYamlFilesInCore
+   */
+  public function testYamlFiles($file) {
+    $data = file_get_contents($file);
+    try {
+      $this->assertEquals(YamlSymfony::decode($data), YamlPecl::decode($data));
+    }
+    catch (InvalidDataTypeException $e) {
+      // Provide file context to the failure so the exception message is useful.
+      $this->fail("Exception thrown parsing $file:\n" . $e->getMessage());
+    }
+  }
+
+  /**
+   * Data provider that lists all YAML files core.
+   * @return array
+   */
+  public function providerYamlFilesInCore() {
+    $files = [];
+    $dirs = new \RecursiveIteratorIterator(new \RecursiveDirectoryIterator(__DIR__ . '/../../../../../', \RecursiveDirectoryIterator::FOLLOW_SYMLINKS));
+    foreach ($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;
+  }
+}
+
+class YamlStub extends Yaml {
+
+  public static function getSerializer() {
+    return '\Drupal\Tests\Component\Serialization\YamlParserProxy';
+  }
+}
+
+class YamlParserProxy implements SerializationInterface {
+
+  /**
+   * @var \Drupal\Component\Serialization\SerializationInterface
+   */
+  static $mock;
+
+  public static function setMock($mock) {
+    static::$mock = $mock;
+  }
+
+  public static function encode($data) {
+    return static::$mock->encode($data);
+  }
+
+  public static function decode($raw) {
+    return static::$mock->decode($raw);
+  }
+
+  public static function getFileExtension() {
+    return static::$mock->getFileExtension();
+  }
 }