Revert "Remove Expect from core library."

tests/language/property_field_override_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.
 
-import "package:expect/expect.dart";
-
 // Test overriding a getter property with a field.
 class A {
   int get v;  // Abstract.
 
   int a() {
     return v - 1;
   }
 }
 
 class B extends A {
   int v = 3;
 
   int b() {
     return ++v;
   }
 }
 
 main() {
   var x = new B();
   Expect.equals(3, x.v);
   Expect.equals(2, x.a());
   Expect.equals(4, x.b());
 }