Remove Expect from core library.

tests/language/field_override2_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.
 
 // Test that we are accessing the right field in a method of a super
 // class, when that field is overridden.
 
+import "package:expect/expect.dart";
+
 class A {
   final a = [42];
   foo() => a[0];
 }
 
 class B extends A {
   final a = new Map();
 }
 
 main() {
   Expect.equals(null, new B().foo());
   Expect.equals(42, new A().foo());
 }