Revert "Remove Expect from core library."

tests/language/instance_field_initializer_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.
 
-import "package:expect/expect.dart";
-
 class A {
   int x = 1;
   A() {}
   A.reassign() : x = 2 {}
   A.reassign2(this.x) {}
 }
 
 class B extends A {
   B() : super() {}
   B.reassign() : super.reassign() {}
   B.reassign2() : super.reassign2(3) {}
 }
 
 class InstanceFieldInitializerTest {
   static testMain() {
     Expect.equals(1, new A().x);
     Expect.equals(2, new A.reassign().x);
     Expect.equals(3, new A.reassign2(3).x);
 
     Expect.equals(1, new B().x);
     Expect.equals(2, new B.reassign().x);
     Expect.equals(3, new B.reassign2().x);
   }
 }
 
 main() {
   InstanceFieldInitializerTest.testMain();
 }