gracefully degrade if package map input file cannot be watched

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 420 matching lines @@
     } else {
       beginComputePackageMap();
       OptimizingPubPackageMapInfo packageMapInfo;
       ServerPerformanceStatistics.pub.makeCurrentWhile(() {
         packageMapInfo =
             _packageMapProvider.computePackageMap(folder, info.packageMapInfo);
       });
       endComputePackageMap();
       for (String dependencyPath in packageMapInfo.dependencies) {
         Resource resource = resourceProvider.getResource(dependencyPath);
-        if (resource is File) {
+        if (resource is File && resource.exists) {

[Paul Berry, 2015/06/22 21:19:31]

This introduces a race condition, because it's possible that the file will be
deleted between the call to .exists here and the existence check that presumably
happens inside the watcher.

It would be better to simply handle the error condition from the file watcher. 
Not only would that eliminate the race condition, it would avoid performing a
redundant existence check in the common scenario where we *aren't* racing with
some other process.

[danrubel, 2015/06/22 21:54:46]

I tried doing that in an earlier iteration of this CL, but could not convince
myself that it would properly handle file system errors without masking problems
in _recomputePackageUriResolver. Based on our conversation I reworked this in
another CL : https://codereview.chromium.org/1199083002

           info.dependencySubscriptions.add(resource.changes
               .listen((WatchEvent event) {
             if (info.packageMapInfo != null &&
                 info.packageMapInfo.isChangedDependency(
                     dependencyPath, resourceProvider)) {
               _recomputePackageUriResolver(info);
             }
           }));
         }
       }
@@ skipping 375 matching lines @@
     return excludes(resource.path);
   }
 
   /**
    * Returns `true` if [path] is the pubspec file of this context.
    */
   bool isPubspec(String path) {
     return path == pubspecPath;
   }
 }