Fix a bug with skipped groups.

lib/src/backend/group.dart

 // Copyright (c) 2015, 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.
 
 library test.backend.group;
 
 import 'metadata.dart';
 import 'operating_system.dart';
 import 'group_entry.dart';
 import 'test.dart';
@@ skipping 26 matching lines @@
 
   Group(this.name, Iterable<GroupEntry> entries, {Metadata metadata,
           Test this.setUpAll, Test this.tearDownAll})
       : entries = new List<GroupEntry>.unmodifiable(entries),
         metadata = metadata == null ? new Metadata() : metadata;
 
   Group forPlatform(TestPlatform platform, {OperatingSystem os}) {
     if (!metadata.testOn.evaluate(platform, os: os)) return null;
     var newMetadata = metadata.forPlatform(platform, os: os);
     var filtered = _map((entry) => entry.forPlatform(platform, os: os));
-    if (filtered.isEmpty) return null;
+    if (filtered.isEmpty && !entries.isEmpty) return null;
     return new Group(name, filtered,
         metadata: newMetadata, setUpAll: setUpAll, tearDownAll: tearDownAll);
   }
 
   Group filter(bool callback(Test test)) {
     var filtered = _map((entry) => entry.filter(callback));
-    if (filtered.isEmpty) return null;
+    if (filtered.isEmpty && !entries.isEmpty) return null;
     return new Group(name, filtered,
         metadata: metadata, setUpAll: setUpAll, tearDownAll: tearDownAll);
   }
 
   /// Returns the entries of this group mapped using [callback].
   ///
   /// Any `null` values returned by [callback] will be removed.
   List<GroupEntry> _map(GroupEntry callback(GroupEntry entry)) {
     return entries
         .map((entry) => callback(entry))
         .where((entry) => entry != null)
         .toList();
   }
 }