Add check to watcher, to avoid null-error if unknown path is removed.

BUG=
R=kevmoo@google.com

Review URL: https://codereview.chromium.org//428493004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/watcher@38846 260f80e4-7a28-3924-810f-c04153c831b5

pkgs/watcher/lib/src/path_set.dart

diff --git a/pkgs/watcher/lib/src/path_set.dart b/pkgs/watcher/lib/src/path_set.dart
index 01d4208..e9f7d32 100644
--- a/pkgs/watcher/lib/src/path_set.dart
+++ b/pkgs/watcher/lib/src/path_set.dart
@@ -71,7 +71,7 @@
         // the next level.
         var part = parts.removeFirst();
         var entry = dir[part];
-        if (entry.isEmpty) return new Set();
+        if (entry == null || entry.isEmpty) return new Set();
 
         partialPath = p.join(partialPath, part);
         var paths = recurse(entry, partialPath);