Port the remaining of dart:coreimpl to the unified corelib.

corelib/unified/coreimpl/expando.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.
 
-interface XMLDocument extends Document, XMLElement
-    default XMLDocumentWrappingImplementation {
+class ExpandoImplementation<T> implements Expando<T> {
+  final String name;
 
-  /** WARNING: Currently this doesn't work on Dartium (issue 649). */
-  XMLDocument.xml(String xml);
+  const ExpandoImplementation([this.name]);
 
-  XMLDocument clone(bool deep);
+  external T operator[](Object object);
+  external void operator[]=(Object object, T value);
+  external String toString();
 }