Revert "Remove Expect from core library."

tests/compiler/dart2js_foreign/native_property_test.dart

 // 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.
 
 // Properties on hidden native classes.
 
-import "package:expect/expect.dart";
 import 'native_metadata.dart';
 
 @Native("*A")
 class A {
 
   // Setters and getters should be similar to these methods:
   @Native('return this._x;')
   int getX();
 
   @Native('this._x = value;')
@@ skipping 43 matching lines @@
   Expect.equals(5, a.getX());
 
   a.X = 10;
   a.Y = 20;
   a.Z = 30;
 
   Expect.equals(10, a.X);
   Expect.equals(20, a.Y);
   Expect.equals(30, a.Z);
 }