Revert "Remove Expect from core library."

tests/language/third_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.
 // Third dart test program.
 // @static-clean
 
-import "package:expect/expect.dart";
-
 class A extends B {
   var a;
   static var s;
 
   static foo() {
     return s;
   }
 
   A(x, y) : super(y), a = x { }
 
@@ skipping 27 matching lines @@
     var b = new B(3);
     A.s = 4;
     B.s = 5;
     Expect.equals(26, a.value() + b.value());
  }
 }
 
 main() {
   ThirdTest.testMain();
 }