Check for FileSystemEntityType.notFound (dart-lang/watcher#80)

The Dart breaking change <dart-lang/sdk#40706>
will change the timestamps on the notFound object from null to the epoch
timestamp.

pkgs/watcher/lib/src/stat.dart

@@ -28,5 +28,6 @@ Future<DateTime> modificationTime(String path) async {
   }
 
   final stat = await FileStat.stat(path);
+  if (stat.type == FileSystemEntityType.notFound) return null;
   return stat.modified;
 }