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

corelib/unified/core/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.
 
 /**
  * An [Expando] allows adding new properties to objects.
  */
-interface Expando<T> default _Expando<T> {
+interface Expando<T> default ExpandoImplementation<T> {
 
   /**
    * The name of the this [Expando] as passed to the constructor. If
    * no name was passed to the constructor, the name is [null].
    */
   final String name;
 
   /**
    * Creates a new [Expando]. The optional name is only used for
    * debugging purposes and creating two different (non-const)
@@ skipping 10 matching lines @@
   T operator [](Object object);
 
   /**
    * Sets the value of this [Expando]'s property on the given
    * object. Properties can effectively be removed again by setting
    * their value to null.
    */
   void operator []=(Object object, T value);
 
 }