Revert "Remove Expect from core library."

tests/language/loop_hoist_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";
-
 class A {
   num x;
   A() : this.x = 0;
 
   void bar(){
     // dart2js hoisted the this.x out of the loop, and missed that setX would
     // change the value.
     for (int i = 1; i < 3; i++) {
       setX(499);
       foo(x);
       break;
     }
   }
   setX(x) => this.x = x;
 }
 
 var saved;
 foo(x) => saved = x;
 
 main() {
   A a = new A();
   for (int i = 0; i < 1; i++) {
     a.bar();
   }
   Expect.equals(499, saved);
 }