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.
+
+/**
+ * An [Expando] allows adding new properties to objects.
+ */
+interface Expando<T> default _Expando<T> {
+
+  /**
+   * The name of the this [Expando] as passed to the constructor. If
+   * no name was passed to the constructor, the name is [null].
+   */
+  final String name;
+
+  /**
+   * Creates a new [Expando]. The optional name is only used for
+   * debugging purposes and creating two different (non-const)
+   * [Expando]s with the same name does not

[Ivan Posva, 2012/06/11 13:54:37]

short comment

[kasperl, 2012/06/12 05:16:05]

Done.

+   */
+  const Expando([String name]);
+
+  /**
+   * Gets the value of this [Expando]'s property on the given
+   * object. If the object hasn't been expanded, the method returns
+   * [null].
+   */
+  T operator [](Object object);
+
+  /**
+   * Sets the value of this [Expando]'s property on the given
+   * object. Properties can effectively be removed again by setting
+   * their value to null.
+   */
+  void operator []=(Object object, T value);
+
+}