gracefully degrade if package map input file cannot be watched

pkg/analysis_server/lib/src/context_manager.dart

Index: pkg/analysis_server/lib/src/context_manager.dart
diff --git a/pkg/analysis_server/lib/src/context_manager.dart b/pkg/analysis_server/lib/src/context_manager.dart
index c4c7e1f71d0676af54689de14f06b54f66b35da4..654bcbe134cfd6b37738e2db872db1d4570c7d5d 100644
--- a/pkg/analysis_server/lib/src/context_manager.dart
+++ b/pkg/analysis_server/lib/src/context_manager.dart
@@ -438,7 +438,7 @@ abstract class ContextManager {
       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 &&