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

corelib/src/implementation/expando.dart

Index: corelib/src/implementation/expando.dart
diff --git a/corelib/src/implementation/expando.dart b/corelib/src/implementation/expando.dart
new file mode 100644
index 0000000000000000000000000000000000000000..44f791ecc91816eaae93c2a9ed85c03f1b011fd9
--- /dev/null
+++ b/corelib/src/implementation/expando.dart
@@ -0,0 +1,11 @@
+// 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.
+
+class ExpandoImplementation<T> implements Expando<T> {
+  final String name;
+  const ExpandoImplementation([String this.name]);
+  String toString() => "Expando:$name";
+  external T operator [](Object object);
+  external void operator []=(Object object, T value);
+}