Provide an Expando implementation for the VM.

corelib/src/corelib_sources.gypi

 # Copyright (c) 2011, 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.
 
 {
   'sources': [
     'bool.dart',
     'collection.dart',
     'comparable.dart',
     'date.dart',
     'double.dart',
     'duration.dart',
     'exceptions.dart',
+    'expando.dart',
     'expect.dart',
     'future.dart',
     'function.dart',
     'int.dart',
     'hashable.dart',
     'iterable.dart',
     'iterator.dart',
     'math.dart',
     'map.dart',
     'list.dart',
     'num.dart',
     'options.dart',
     'pattern.dart',
     'queue.dart',
     'regexp.dart',
     'set.dart',
     'stopwatch.dart',
     'string.dart',
     'strings.dart',
     'string_buffer.dart',
   ],
 }