rework packageMapInfo dependency watching to avoid race condition

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 && resource.exists) {
-          info.dependencySubscriptions.add(resource.changes
-              .listen((WatchEvent event) {
+        if (resource is File) {
+          StreamSubscription<WatchEvent> subscription;
+          subscription = resource.changes.listen((WatchEvent event) {
             if (info.packageMapInfo != null &&
                 info.packageMapInfo.isChangedDependency(
                     dependencyPath, resourceProvider)) {
               _recomputePackageUriResolver(info);
             }
-          }));
+          }, onError: (error, StackTrace stackTrace) {
+            // Gracefully degrade if file is or becomes unwatchable
+            _instrumentationService.logException(error, stackTrace);
+            subscription.cancel();
+            info.dependencySubscriptions.remove(subscription);
+          });
+          info.dependencySubscriptions.add(subscription);
         }
       }
       info.packageMapInfo = packageMapInfo;
       if (packageMapInfo.packageMap == null) {
         return null;
       }
       return new PackageMapUriResolver(
           resourceProvider, packageMapInfo.packageMap);
       // TODO(paulberry): if any of the dependencies is outside of [folder],
       // we'll need to watch their parent folders as well.
@@ skipping 367 matching lines @@
     return excludes(resource.path);
   }
 
   /**
    * Returns `true` if [path] is the pubspec file of this context.
    */
   bool isPubspec(String path) {
     return path == pubspecPath;
   }
 }