Revert "Remove Expect from core library."

tests/language/state_mangling2_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";
-
 foo(state) {
   if (state == null) return 0;
   var sum = 0;
   state = inscrutableId(state);
   for (int i = 0; i < state.length; i++) {
     sum += state[i];
   }
   state = inscrutableId(state);
   for (int i = 0; i < state.length; i++) {
     sum += state[i];
@@ skipping 12 matching lines @@
   int length = 3;
   operator [](i) => 1;
 }
 
 main() {
   Expect.equals(12, foo([1, 2, 3]));
   if (inscrutableId(0) == 0) {
     Expect.equals(6, foo(new A()));
   }
 }