Rewrite the pkg/yaml parser.

pkg/yaml/lib/src/yaml_document.dart

+// Copyright (c) 2014, 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 yaml.yaml_document;
+
+import 'dart:collection';
+
+import 'package:source_span/source_span.dart';
+
+import 'yaml_node.dart';
+
+/// A YAML document, complete with metadata.
+class YamlDocument {
+  /// The contents of the document.
+  final YamlNode contents;
+
+  /// The span covering the entire document.
+  final SourceSpan span;
+
+  /// The version directive for the document, if any.
+  final VersionDirective versionDirective;
+
+  /// The tag directives for the document.
+  final List<TagDirective> tagDirectives;
+
+  /// Whether the beginning of the document was implicit (versus explicit via
+  /// `===`).
+  final bool startImplicit;
+
+  /// Whether the end of the document was implicit (versus explicit via `...`).
+  final bool endImplicit;
+
+  /// Users of the library should not use this constructor.
+  YamlDocument.internal(this.contents, this.span, this.versionDirective,
+          List<TagDirective> tagDirectives, {this.startImplicit: false,
+          this.endImplicit: false})
+      : tagDirectives = new UnmodifiableListView(tagDirectives);
+
+  String toString() => contents.toString();
+}
+
+/// A directive indicating which version of YAML a document was written to.
+class VersionDirective {
+  /// The major version number.
+  final int major;
+
+  /// The minor version number.
+  final int minor;
+
+  VersionDirective(this.major, this.minor);
+
+  String toString() => "%YAML $major.$minor";
+}
+
+/// A directive describing a custom tag handle.
+class TagDirective {
+  /// The handle for use in the document.
+  final String handle;
+
+  /// The prefix that the handle maps to.
+  final String prefix;
+
+  TagDirective(this.handle, this.prefix);
+
+  String toString() => "%TAG $handle $prefix";
+}