Revert "Remove Expect from core library."

tests/language/mixin_getter_regression_test.dart

 // Copyright (c) 2013, 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.
 
 // Regression test case for dart2js bug where the getter for y wasn't
 // properly mixed in.
 
-import "package:expect/expect.dart";
-
 class C {
   int x;
   int get y => x;
 }
 
 class E {
   int z = 10;
 }
 
 class D extends E with C {
   int w = 42;
 }
 
 main() {
   var d = new D();
   d.x = 37;
   Expect.equals(37, d.x);
   Expect.equals(10, d.z);
   Expect.equals(42, d.w);
   Expect.equals(37, d.y);
 }