Revert "Remove Expect from core library."

tests/language/operator2_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.
 // Operator dart test program.
 
-import "package:expect/expect.dart";
-
 class Helper {
   int i;
   Helper(int val) : i = val { }
   operator [](int index) {
     return i + index;
   }
   void operator []=(int index, int val) {
     i = val;
   }
 }
 
 class OperatorTest {
   static testMain() {
     Helper obj = new Helper(10);
     Expect.equals(10, obj.i);
     obj[10] = 20;
     Expect.equals(30, obj[10]);
   }
 }
 
 
 main() {
   OperatorTest.testMain();
 }