Revert "Remove Expect from core library."

tests/language/for2_test.dart

 // Copyright (c) 2011, 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 program for testing for statement which captures loop variable.
 
-import "package:expect/expect.dart";
-
 var f;
 
 main() {
   // Capture the loop variable, ensure we capture the right value.
   for (int i = 0; i < 10; i++) {
     if (i == 7) {
       f = () => "i = $i";
     }
   }
   Expect.equals("i = 7", f());
@@ skipping 44 matching lines @@
   // Nested loops with captured variables.
   for (int k = 0; k < 5; k++) {
     for (int i = 0; i < 10; i++) {
       if (k == 3 && i == 7) {
         f = () => "k = $k, i = $i";
       }
     }
   }
   Expect.equals("k = 3, i = 7", f());
 }