Use WeakMap to support Expando if available.

runtime/lib/expando_patch.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.
 
 patch class Expando<T> {
   /* patch */ Expando([String this.name])
       : _data = new List(_minSize),
         _used = 0;
 
   static const _minSize = 8;
@@ skipping 113 matching lines @@
         }
       }
     }
   }
 
   static _checkType(object) {
     if ((object == null) ||
          (object is bool) ||
          (object is num) ||
          (object is String)) {
-      throw new ArgumentError(object);
+      throw new ArgumentError.value(object,
+          "Expandos are not allowed on strings, numbers, booleans or null");
     }
   }
 
   get _size => _data.length;
   get _limit => (3 * (_size ~/ 4));
 
   List _data;
   int _used;  // Number of used (active and deleted) slots.
 }