Revert "Remove Expect from core library."

tests/language/operator6_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.
 
-import "package:expect/expect.dart";
-
 class OperatorTest {
   static int i1, i2;
 
   OperatorTest() {}
 
   static testMain() {
     var op1 = new Operator(1);
     var op2 = new Operator(2);
     Expect.equals(~1, ~op1);
   }
 }
 
 class Operator {
   int value;
 
   Operator(int i) {
     value = i;
   }
 
   operator ~() {
     return ~value;
   }
 }
 
 main() {
   OperatorTest.testMain();
 }