Use patching for Expando so the implementation-specific files are not needed in the SDK.

lib/compiler/implementation/lib/coreimpl_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 file for dart:core classes.
 
 // Patch for Date implementation.
 // TODO(ager): Split out into date_patch.dart and allow #source
 // in patch files?
 patch class DateImplementation {
@@ skipping 181 matching lines @@
     // hidden global flag.
     _next = _re.firstMatch(_str);
     if (_next == null) {
       _done = true;
       return false;
     } else {
       return true;
     }
   }
 }
+
+// Patch for Expando implementation.
+// TODO(ager): Split out into expando_patch.dart and allow #source in
+// patch files?
+patch class ExpandoImplementation<T> {
+
+  patch T operator[](Object object) {
+    var values = Primitives.getProperty(object, _EXPANDO_PROPERTY_NAME);
+    return (values === null) ? null : Primitives.getProperty(values, _getKey());
+  }
+
+  patch void operator[]=(Object object, T value) {
+    var values = Primitives.getProperty(object, _EXPANDO_PROPERTY_NAME);
+    if (values === null) {
+      values = new Object();
+      Primitives.setProperty(object, _EXPANDO_PROPERTY_NAME, values);
+    }
+    Primitives.setProperty(values, _getKey(), value);
+  }
+
+  String _getKey() {
+    String key = Primitives.getProperty(this, _KEY_PROPERTY_NAME);
+    if (key === null) {
+      key = "expando\$key\$${_keyCount++}";
+      Primitives.setProperty(this, _KEY_PROPERTY_NAME, key);
+    }
+    return key;
+  }
+
+  static final String _KEY_PROPERTY_NAME = 'expando\$key';
+  static final String _EXPANDO_PROPERTY_NAME = 'expando\$values';
+  static int _keyCount = 0;
+}