Revert "Remove Expect from core library."

tests/language/numbers_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.
 // Test number types.
 
 library NumbersTest.dart;
-import "package:expect/expect.dart";
 
 class NumbersTest {
   static double testMain() {
     var one = 1;
     Expect.equals(true, one is Object);
     Expect.equals(true, one is num);
     Expect.equals(true, one is int);
     Expect.equals(false, one is double);
 
     var two = 2.0;
@@ skipping 10 matching lines @@
 
     Expect.equals(3.0, result);
     return result;
   }
 }
 
 
 main() {
   NumbersTest.testMain();
 }