Support user generated custom native JS classes.

pkg/js/lib/js.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.
+
+/**
+ * The js library allows Dart library authors to export their APIs to JavaScript
+ * and to define Dart interfaces for JavaScript objects.
+ */
+library js;
+
+/// A metadata annotation that indicates that a Library, Class, or member is
+/// implemented directly in JavaScript. All external members of a class or
+/// library with this annotation implicitly have it as well.
+///
+/// Specifying [name] customizes the JavaScript name to use. By default the
+/// dart name is used. It is not valid to specify a custom [name] for class
+/// instance members.

[sra1, 2015/10/01 20:55:28]

Comment needs examples.

[Jacob, 2015/10/02 20:08:16]

Done.

+class Js {
+  final String name;
+  const Js([this.name]);
+}
+
+/// A metadata annotation used to indicate that the Js object is a anonymous js
+/// object. That is it is created with `new Object()`.

[sra1, 2015/10/01 20:55:28]

Say why this is needed.
Add example.

[Jacob, 2015/10/02 20:08:16]

Removed as we aren't using it yet.

+const anonymous = const _Anonymous();
+class _Anonymous {
+  const _Anonymous();
+}