Revert "Remove Expect from core library."

tests/language/constructor_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 constructors and initializers.
 
-import "package:expect/expect.dart";
-
 class A extends B {
   A(x, y) : super(y), a = x { }
 
   var a;
 }
 
 
 class B {
   var b;
 
@@ skipping 35 matching lines @@
     Expect.equals(22, o.a + o.b + o1.b);
 
     var beta = new Beta(3);
     Expect.equals(3, beta.b);
   }
 }
 
 main() {
   ConstructorTest.testMain();
 }