Reapply changes to Expando patching.

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 ExpandoImplementation<T> {
+patch class Expando<T> {
   /* patch */ T operator[](Object object) {
     _checkType(object);
     var weakProperty = _find(this);
     var list = weakProperty.value;
     var doCompact = false;
     var result = null;
     for (int i = 0; i < list.length; ++i) {
       var key = list[i].key;
       if (key === object) {
         result = list[i].value;
@@ skipping 67 matching lines @@
     }
     var result = _data[i];
     if (doCompact) {
       _data = _data.filter((e) => (e !== null));
     }
     return result;
   }
 
   static List _data;
 }