Make VM Expando use identityHashCode instead of object's hashCode.

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;
   static final _deletedEntry = new _WeakProperty(null, null);
 
   /* patch */ T operator[](Object object) {
     _checkType(object);
 
     var mask = _size - 1;
-    var idx = object.hashCode & mask;
+    var idx = object._identityHashCode & mask;
     var wp = _data[idx];
 
     while (wp != null) {
       if (identical(wp.key, object)) {
         return wp.value;
       } else if (wp.key == null) {
         // This entry has been cleared by the GC.
         _data[idx] = _deletedEntry;
       }
       idx = (idx + 1) & mask;
       wp = _data[idx];
     }
 
     return null;
   }
 
   /* patch */ void operator[]=(Object object, T value) {
     _checkType(object);
 
     var mask = _size - 1;
-    var idx = object.hashCode & mask;
+    var idx = object._identityHashCode & mask;
     var empty_idx = -1;
     var wp = _data[idx];
 
     while (wp != null) {
       if (identical(wp.key, object)) {
         if (value != null) {
           // Update the associated value.
           wp.value = value;
         } else {
           // Mark the entry as deleted.
           _data[idx] = _deletedEntry;
         }
         return;
       } else if ((empty_idx < 0) && identical(wp, _deletedEntry)) {
         empty_idx = idx;  // Insert at this location if not found.
       } else if (wp.key == null) {
         // This entry has been cleared by the GC.
         _data[idx] = _deletedEntry;
         if (empty_idx < 0) {
           empty_idx = idx;  // Insert at this location if not found.
         }
       }
       idx = (idx + 1) & mask;
       wp = _data[idx];
     }
 
     if (value == null) {
       // Not entering a null value. We just needed to make sure to clear an
-      // existing value if it existed. 
+      // existing value if it existed.
       return;
     }
 
     if (empty_idx >= 0) {
       // We will be reusing the empty slot below.
       _used--;
       idx = empty_idx;
     }
 
     if (_used < _limit) {
@@ skipping 56 matching lines @@
       throw new ArgumentError(object);
     }
   }
 
   get _size => _data.length;
   get _limit => (3 * (_size ~/ 4));
 
   List _data;
   int _used;  // Number of used (active and deleted) slots.
 }