Revert "Remove Expect from core library."

tests/language/assign_op_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 assign operators.
 
-import "package:expect/expect.dart";
-
 
 class AssignOpTest {
   AssignOpTest() {}
 
   static testMain() {
     var b = 0;
     b += 1;
     Expect.equals(1, b);
     b *= 5;
     Expect.equals(5, b);
@@ skipping 68 matching lines @@
   }
 
   static var f;
   var instf;
 }
 main() {
   for (int i = 0; i < 2000; i++) {
     AssignOpTest.testMain();
   }
 }