Revert "Remove Expect from core library."

tests/language/operator_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(3, op1 + op2);
     Expect.equals(-1, op1 - op2);
@@ skipping 113 matching lines @@
   }
 
   operator -() {
     return -value;
   }
 }
 
 main() {
   OperatorTest.testMain();
 }