Revert "Remove Expect from core library."

tests/language/constructor_default_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 default constructors.
 
-import "package:expect/expect.dart";
-
 class A {
   A() : a = 499;
 
   var a;
 }
 
 class B extends A {
   B() { Expect.equals(499, a); }
 }
 
 main() {
   new B();
 }