Revert "Remove Expect from core library."

tests/language/naming_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.
 
-import "package:expect/expect.dart";
-
 class A {
   A() { NamingTest.count++; }
   foo(a, b) {
     Expect.equals(1, a);
     Expect.equals(2, b);
   }
 }
 
 class MyException {
   MyException() {}
@@ skipping 423 matching lines @@
         // We check for both exceptions because the exact exception to
         // throw is hard to compute on some browsers.
         (e) => e is NoSuchMethodError);
   }
 }
 
 main() {
   NamingTest.testMain();
   Naming2Test.main(null);
 }