Revert "Remove Expect from core library."

tests/language/closure6_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";
-
 // Test that captured final variables are correctly mangled.
 
 class A {
   foo() {
     length() => 400;
     final box_0 = 28;
     var x = 29;
     var f = () => length() + box_0 + x + bar();
     return f();
   }
   bar() => 42;
 }
 
 main() {
   Expect.equals(499, new A().foo());
 }