Fix dartbug.com/11757:

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([this.name])
       : _data = new List(_minSize),
         _used = 0;
 
   static const _minSize = 8;
@@ skipping 96 matching lines @@
     }
     new_size = (new_size < _minSize) ? _minSize : new_size;
 
     // Reset the mappings to empty so that we can just add the existing
     // valid entries.
     _data = new List(new_size);
     _used = 0;
 
     for (var i = 0; i < old_data.length; i++) {
       var entry = old_data[i];
-      if ((entry != null) && (entry.key != null)) {
-        this[entry.key] = entry.value;
+      if (entry != null) {
+        // Ensure that the entry.key is not cleared between checking for it and
+        // inserting it into the new table.
+        var val = entry.value;
+        var key = entry.key;
+        if (key != null) {
+          this[key] = val;
+        }
       }
     }
   }
 
   static _checkType(object) {
     if ((object == null) ||
          (object is bool) ||
          (object is num) ||
          (object is String)) {
       throw new ArgumentError(object);
     }
   }
 
   get _size => _data.length;
   get _limit => (3 * (_size ~/ 4));
 
   List _data;
   int _used;  // Number of used (active and deleted) slots.
 }