Revert "Remove Expect from core library."

tests/language/double_modulo_test.dart

 // Copyright (c) 2013, 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 optimization of modulo operator on Double.
 
-import "package:expect/expect.dart";
-
 main() {
   double k = -0.33333;
   double firstResPos = doMod(k, 1.0);
   double firstResNeg = doMod(k, -1.0);
   for (int i = 0; i < 5000; i++) {
     Expect.equals(firstResPos, doMod(k, 1.0));
     Expect.equals(firstResNeg, doMod(k, -1.0));
   }
 }
 
 doMod(a, b) => a % b;