Add test for 'refresh()' and clean up other tests.

pkg/analysis_server/lib/src/single_context_manager.dart

 // Copyright (c) 2016, 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 analysis_server.src.single_context_manager;
 
 import 'dart:async';
 import 'dart:core' hide Resource;
 import 'dart:math' as math;
 
@@ skipping 128 matching lines @@
 
   @override
   bool isInAnalysisRoot(String path) {
     return _isContainedIn(includedPaths, path) &&
         !_isContainedIn(excludedPaths, path);
   }
 
   @override
   void refresh(List<Resource> roots) {
     if (context != null) {
-      // TODO(brianwilkerson) Not sure whether this is right.
       callbacks.removeContext(contextFolder, null);
       context.dispose();
       context = null;
       contextFolder = null;
       if (watchSubscription != null) {
         watchSubscription.cancel();
         watchSubscription = null;
       }
       setRoots(includedPaths, excludedPaths, packageRoots);
     }
@@ skipping 314 matching lines @@
   static List<Resource> _getChildrenSafe(Folder folder) {
     try {
       return folder.getChildren();
     } on FileSystemException {
       // The folder either doesn't exist or cannot be read.
       // Either way, there are no children.
       return const <Resource>[];
     }
   }
 }