Revert "Remove Expect from core library."

tests/language/field_override_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.
 // Test overriding of fields.
 
-import "package:expect/expect.dart";
-
 
 class A {}
 class B1 extends A {}
 class B2 extends A {}
 
 class Super {
   Super() : super();
 
   B1 field;
 }
@@ skipping 20 matching lines @@
   val2.field = new A();
   Expect.equals(true, val2.field is A);
   Expect.equals(false, val2.field is B1);
   Expect.equals(false, val2.field is B2);
 
   Super val3 = new Super();
   val3.field = new B1();
   Expect.equals(true, val3.field is B1);
   Expect.equals(false, val3.field is B2);
 }