Add a test for tags to configuration_test.

lib/src/runner/configuration.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.
 
 import 'dart:io';
 
 import 'package:boolean_selector/boolean_selector.dart';
 import 'package:collection/collection.dart';
 import 'package:glob/glob.dart';
 import 'package:path/path.dart' as p;
 
 import '../backend/metadata.dart';
+import '../backend/operating_system.dart';

[kevmoo, 2016/03/01 06:01:12]

Unneeded import...

[nweiz, 2016/03/01 19:24:49]

Done.

 import '../backend/platform_selector.dart';
 import '../backend/test_platform.dart';
 import '../frontend/timeout.dart';
 import '../util/io.dart';
 import '../utils.dart';
 import 'configuration/args.dart' as args;
 import 'configuration/load.dart';
 import 'configuration/values.dart';
 
 /// A class that encapsulates the command-line configuration of the test runner.
@@ skipping 250 matching lines @@
         filename: other._filename ?? _filename,
         includeTags: includeTags.intersection(other.includeTags),
         excludeTags: excludeTags.union(other.excludeTags),
         addTags: other.addTags.union(addTags),
         tags: mergeMaps(tags, other.tags,
             value: (config1, config2) => config1.merge(config2)),
         onPlatform: mergeMaps(onPlatform, other.onPlatform,
             value: (config1, config2) => config1.merge(config2)));
   }
 }