Revert "Remove Expect from core library."

tests/language/context2_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.
 // Dart test for capturing.
 
-import "package:expect/expect.dart";
-
 // Regression test for issue 5991015.
 // @static-clean
 
 class V {
 
   notCalled(Function x) { return x(); }
 
   foofoo(x) { return x; }
 
   hoop(input, n) {
     while (n-- > 0) {
       Expect.equals(5, input);
       continue;  // This continue statement must properly unchain the context.
       switch (input) {
         case 3:
           var values = foofoo;
           notCalled(() => values(input));
       }
     }
   }
 }
 
 main() {
   new V().hoop(5, 3);
 }