Revert "Remove Expect from core library."

tests/language/modulo_test.dart

 // Copyright (c) 2013, 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 optimization of modulo operator on Smi.
 
-import "package:expect/expect.dart";
-
 
 main() {
   for (int i = -3000; i < 3000; i++) {
     Expect.equals(i % 256, foo(i));
     Expect.equals(i % -256, boo(i));
     try {
       hoo(i);
       Expect.fail("Exception expected.");
     } catch (e) {}
   }
 }
 
 foo(i) {
   return i % 256;  // This will get optimized to AND instruction.
 }
 
 
 boo(i) {
   return i % -256;
 }
 
 
 hoo(i) {
   return i % 0;
 }