Revert "Remove Expect from core library."

tests/language/library1_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.
 // Dart test program for testing libraries.
 
 library Library1Test.dart;
-import "package:expect/expect.dart";
 import "library1_lib.lib";
 
 main() {
   Library1Test.testMain();
 }
 
 class Library1Test {
   static testMain() {
     var a = new A();
     String s = a.foo();
     Expect.equals(s, "foo-rty two");
   }
 }