Revert "Remove Expect from core library."

tests/language/default_class_implicit_constructor_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";
-
 // An abstract class with a redirecting factory to a class with no declared
 // constructor should use the implicit default constructor.
 
 abstract class A {
   factory A() = B;
 }
 
 class B implements A {
 }
 
 main() {
   var val = new A();
   Expect.equals(true, val is A);
   Expect.equals(true, val is B);
 }