Revert "Remove Expect from core library."

tests/language/library1_negative_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.
 
 // This test should fail to load because we are importing two libraries
 // which define the same top level name foo, and we are referring to the name.
 
 library Library1NegativeTest.dart;
-import "package:expect/expect.dart";
 import "library1.dart";  // Defines top level variable 'foo'
 import "library2.dart";  // Defines top level variable 'foo'
 
 
 main() {
   Expect.equals(0, foo); // Reference ambiguous name 'foo'
 }