Revert "Remove Expect from core library."

tests/compiler/dart2js_extra/closure_capture5_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() {
   var fs = [];
   for (var x = 1; x <= 3; x++) {
     fs.add(fun() { return x; });
   }
   Expect.equals(3, fs.length);
   Expect.equals(1, fs[0]());
   Expect.equals(2, fs[1]());
   Expect.equals(3, fs[2]());
 }
@@ skipping 50 matching lines @@
   Expect.equals(0, g());
 }
 
 main() {
   closure0();
   closure1();
   closure2();
   closure3();
   closure4();
 }