Add --hide-package-warnings option.

tests/compiler/dart2js/in_user_code_test.dart

 // Copyright (c) 2014, 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 that the debugging helper [Compiler.inUserCode] works as intended.
+// Test that the helper [Compiler.inUserCode] works as intended.
 
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 
 import 'memory_compiler.dart';
 
 const SOURCE = const {
   'main.dart': """
 library main;
 
@@ skipping 72 matching lines @@
         'package:sup/boz.dart': true,
         'dart:core': false});
   test([Uri.parse('dart:async'), Uri.parse('package:sub/bar.dart')],
        {'package:sub/bar.dart': true,
         'package:sub/baz.dart': true,
         'package:sup/boz.dart': false,
         'dart:core': true,
         'dart:async': true});
 }