Add experimental expando support.

corelib/src/expando.dart

+// Copyright (c) 2012, 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.
+
+interface Expando<T> default ExpandoImplementation<T> {

[Ivan Posva, 2012/06/11 11:18:33]

Private implementation?

+
+  final String name;
+
+  /**
+   *
+   */
+  Expando([String name]);
+
+  /**
+   *
+   */
+  T operator [](Object object);

[Ivan Posva, 2012/06/11 11:18:33]

What is the result if the value does not exist? Is it legal to associate null
with a key, if so we will also need a has.

+
+  /**
+   *
+   */
+  void operator []=(Object object, T value);
+

[Ivan Posva, 2012/06/11 11:18:33]

Deleting a value is something we discussed. In the JS case this might mean to
set the property to null or a deleted sentinel.

+}