Fix an incomplete comment.

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;
@@ skipping 293 matching lines @@
         patterns: patterns,
         platforms: platforms,
         paths: paths,
         filename: filename,
         chosenPresets: chosenPresets,
         includeTags: includeTags,
         excludeTags: excludeTags,
         addTags: addTags,
 
         // Make sure we pass [chosenPresets] to the child configurations as
-        // well. This ensures that 
+        // well. This ensures that tags and platforms can have preset-specific
+        // behavior.
         tags: _withChosenPresets(tags, chosenPresets),
         onPlatform: _withChosenPresets(onPlatform, chosenPresets),
         presets: _withChosenPresets(presets, chosenPresets));
 
     if (chosenPresets == null) return _unresolved();
     chosenPresets = new Set.from(chosenPresets);
 
     if (presets == null) return _unresolved();
     presets = new Map.from(presets);
 
@@ skipping 222 matching lines @@
 
   /// Merges two maps whose values are [Configuration]s.
   ///
   /// Any overlapping keys in the maps have their configurations merged in the
   /// returned map.
   Map<Object, Configuration> _mergeConfigMaps(Map<Object, Configuration> map1,
           Map<Object, Configuration> map2) =>
       mergeMaps(map1, map2,
           value: (config1, config2) => config1.merge(config2));
 }