Revert "Remove Expect from core library."

tests/language/super_setter_interceptor_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 correctly intercept super getter and setter calls.
 
-import "package:expect/expect.dart";
-
 var expected;
 
 class A {
   set length(a) { Expect.equals(expected, a); }
   get length => 41;
 }
 
 class B extends A {
   test() {
     expected = 42;
@@ skipping 10 matching lines @@
     Expect.equals(41, super.length--);
     Expect.equals(41, super.length);
   }
 }
 
 main() {
   // Ensures the list class is instantiated.
   print([]);
   new B().test();
 }