Revert "Remove Expect from core library."

tests/language/class_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";
-
 // Tests basic classes and methods.
 class ClassTest {
   ClassTest() {}
 
   static testMain() {
     var test = new ClassTest();
     test.testSuperCalls();
     test.testVirtualCalls();
     test.testStaticCalls();
     test.testInheritedField();
@@ skipping 146 matching lines @@
   }
 
   Sub() : super(42) {
     y_ = 0;
   }
 }
 
 main() {
   ClassTest.testMain();
 }