Add a parseDartString method to pkg/analyzer.

pkg/analyzer_experimental/lib/src/string_source.dart

+// Copyright (c) 2013, 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.string_source;
+
+import 'generated/source.dart';
+
+/// An implementation of [Source] that's based on an in-memory Dart string.
+class StringSource implements Source {
+  final String _contents;
+  final String fullName;
+  final int modificationStamp;
+
+  StringSource(this._contents, this.fullName)
+      : modificationStamp = new DateTime.now().millisecondsSinceEpoch;
+
+  bool operator==(Object object) => object is StringSource &&
+      object._contents == _contents && object._name == _name;
+
+  bool exists() => true;
+
+  void getContents(Source_ContentReceiver receiver) =>
+      receiver.accept2(_contents, modificationStamp);
+
+  String get encoding => throw UnsupportedError("StringSource doesn't support "
+      "encoding.");
+
+  String get shortName => fullName;
+
+  String get uriKind => throw UnsupportedError("StringSource doesn't support "
+      "uriKind.");
+
+  int get hashCode => _contents.hashCode ^ _name.hashCode;
+
+  bool get isInSystemLibrary => false;
+
+  Source resolveRelative(Uri relativeUri) => throw UnsupportedError(
+      "StringSource doesn't support resolveRelative.");
+}