Guard against exception when accessing folder

pkg/analysis_server/lib/src/context_manager.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.
 
 library context.directory.manager;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analysis_server/src/analysis_server.dart';
@@ skipping 241 matching lines @@
       Folder contextFolder, UriResolver packageUriResolver);
 
   /**
    * Resursively adds all Dart and HTML files to the [changeSet].
    */
   void _addPreviouslyExcludedSources(_ContextInfo info, ChangeSet changeSet,
       Folder folder, List<String> oldExcludedPaths) {
     if (info.excludesResource(folder)) {
       return;
     }
-    List<Resource> children = folder.getChildren();
+    List<Resource> children;
+    try {
+      children = folder.getChildren();
+    } on FileSystemException catch (exception) {
+      // The folder no longer exists, or cannot be read, to there's nothing to
+      // do.
+      return;
+    }
     for (Resource child in children) {
       String path = child.path;
       // add files, recurse into folders
       if (child is File) {
         // ignore if should not be analyzed at all
         if (!_shouldFileBeAnalyzed(child)) {
           continue;
         }
         // ignore if was not excluded
         bool wasExcluded = _isExcludedBy(oldExcludedPaths, path) &&
@@ skipping 430 matching lines @@
     return excludes(resource.path);
   }
 
   /**
    * Returns `true` if [path] is the pubspec file of this context.
    */
   bool isPubspec(String path) {
     return path == pubspecPath;
   }
 }