Revert "Remove Expect from core library."

tests/language/div_by_zero_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 to test integer div by zero.
 
-import "package:expect/expect.dart";
-
 class DivByZeroTest {
 
   static double divBy(int a, int b) {
     var result = a/b;
     return 1.0 * result;
   }
 
   static bool moustacheDivBy(int a, int b) {
     var val = null;
     try {
       val = a~/b;
     } catch (e) {
       return true;
     }
     print("Should not have gotten: $val");
     return false;
   }
 
   static void testMain() {
     Expect.isTrue(divBy(0, 0).isNaN);
     Expect.isTrue(moustacheDivBy(0, 0));
   }
 }
 
 main() {
   DivByZeroTest.testMain();
 }