Revert "Remove Expect from core library."

tests/language/closure_break_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 for closures.
 
-import "package:expect/expect.dart";
-
 class ClosureBreak {
   ClosureBreak(this.field);
   int field;
 }
 
 class ClosureBreakTest {
   static testMain() {
     var o1 = new ClosureBreak(3);
     String newstr = "abcdefgh";
     foo() {
@@ skipping 24 matching lines @@
       break L1;
     }
     foo();
     Expect.equals(4, o1.field);
   }
 }
 
 main() {
   ClosureBreakTest.testMain();
 }