Provide an Expando implementation for the VM.

runtime/lib/weak_property.dart

Index: runtime/lib/weak_property.dart
diff --git a/runtime/lib/weak_property.dart b/runtime/lib/weak_property.dart
index b3e4bfd23c3605a47d66dbb57ddc3d2d6ef9ffcf..a9348c36add9868e1fe51b0bb9c4dbfc29531a80 100644
--- a/runtime/lib/weak_property.dart
+++ b/runtime/lib/weak_property.dart
@@ -2,14 +2,16 @@
 // 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.
 
-class WeakProperty {
-  factory WeakProperty(key, value) => _new(key, value);
+class _WeakProperty {
+  factory _WeakProperty(key, value) => _new(key, value);
 
   get key() => _getKey();
   get value() => _getValue();
+  set value(value) => _setValue(value);
 
-  static WeakProperty _new(key, value) native "WeakProperty_new";
+  static _WeakProperty _new(key, value) native "WeakProperty_new";
 
   _getKey() native "WeakProperty_getKey";
   _getValue() native "WeakProperty_getValue";
+  _setValue(value) native "WeakProperty_setValue";
 }