Revert "Remove Expect from core library."

tests/language/override_method_with_field_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";
-
 // Test overriding a method with a field.
 
 class Super {
   Super() : super();
 
   instanceMethod() => 42;
 }
 
 class Sub extends Super {
   Sub() : super();
 
   var instanceMethod = 87;  /// 01: compile-time error
 
   superInstanceMethod() => super.instanceMethod();
 }
 
 main() {
   var s = new Sub();
   Super sup = s;
   Sub sub = s;
   print(s.instanceMethod);
   Expect.equals(42, s.superInstanceMethod());
   Expect.equals(42, sup.superInstanceMethod()); /// 02: static type warning
   Expect.equals(42, sub.superInstanceMethod());
 }