Revert "Remove Expect from core library."

tests/language/comparison_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 comparison operators.
 
-import "package:expect/expect.dart";
-
 class Helper {
   static bool STRICT_EQ(a, b) {
     return identical(a, b);
   }
 
   static bool STRICT_NE(a, b) {
     return !identical(a, b);
   }
 
   static bool EQ(a, b) {
@@ skipping 298 matching lines @@
     Expect.isFalse(Helper.EQ(null, 1.1));
 
     // TODO(srdjan): Clarify behaviour of greater/less comparisons
     // between numbers and non-numbers.
   }
 }
 
 main() {
   ComparisonTest.testMain();
 }