Revert "Remove Expect from core library."

tests/language/gvn_field_access_test.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.
 
-import "package:expect/expect.dart";
-
 class A {
   var y = 0;
   foo(x) {
     var t = this.y;
     if (t < x) {
       for (int i = this.y; i < x; i++) y++;
     }
     // dart2js was reusing the 't' from above.
     return this.y;
   }
 }
 
 void main() {
   Expect.equals(3, new A().foo(3));
 }