Revert "Remove Expect from core library."

tests/compiler/dart2js_extra/closure_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";
-
 closure0() {
   // TODO(floitsch): remove name from functions.
   var f = fun() { return 499; };
   Expect.equals(499, f());
 }
 
 class A {
   closure1() {
     var f = fun() { return 499; };
     Expect.equals(499, f());
@@ skipping 21 matching lines @@
   Expect.equals(499, applyFun2(fun(x, y) { return x + y; }));
 }
 
 main() {
   closure0();
   new A().closure1();
   closure2();
   closure3();
   closure4();
 }