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.

[sra1, 2012/06/12 07:10:26]

It does not look like adding properties to an object.
I can't start writing obj.foo or obj[foo] where foo is the Expando.
I have to write foo[obj] which looks more like an identity map.
So why is this called 'Expando' rather than 'IdentityMap' or something similar?

Ultimately this is a map from instances to values with some lifetime guarantees.
The values attached to an object via an Expando will be GC-ed if the object is
GC-ed and is the only reference to the value is via the Expando.  It is an
implementation detail that the information directly hangs off the instance.

+ */
+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

[sra1, 2012/06/12 07:10:26]

'only used' - I'm confused.
If I create two const [Expando]s with the same name, the behaviour is not
(obviously) specified in this one-sentence comment.

Please spell out the const and non-const cases.
Significant different behaviors deserve separate sentences.

The comment must at least explain the coding pattern for how a package should
use the API to add a field/expando that does not conflict with other packages.

+   * debugging purposes and creating two different (non-const)
+   * [Expando]s with the same name yields two [Expando]s that work on
+   * different properties of the objects they are used on.
+   */
+  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].

[sra1, 2012/06/12 07:10:26]

Explain when the call will fail,

+   */
+  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);
+
+}