Add minimal yaml support to the engine

pkg/analyzer/lib/task/yaml.dart

+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library analyzer.task.yaml;
+
+import 'package:analyzer/src/generated/error.dart';
+import 'package:analyzer/src/generated/source.dart';
+import 'package:analyzer/task/model.dart';
+import 'package:yaml/yaml.dart';
+
+/**
+ * The result of parsing a YAML file.
+ */
+final ResultDescriptor<YamlDocument> YAML_DOCUMENT =
+    new ResultDescriptor<YamlDocument>('YAML_DOCUMENT', null);
+
+/**
+ * The analysis errors associated with a [Source] representing a YAML file.
+ */
+final ListResultDescriptor<AnalysisError> YAML_ERRORS =
+    new ListResultDescriptor<AnalysisError>(
+        'YAML_ERRORS', AnalysisError.NO_ERRORS);
+
+/**
+ * The line information for a [Source].
+ */
+final ResultDescriptor<LineInfo> YAML_LINE_INFO =
+    new ResultDescriptor<LineInfo>('YAML_LINE_INFO', null);