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

pkg/watcher/lib/src/path_set.dart

 // Copyright (c) 2013, 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 watcher.path_dart;
 
 import 'dart:collection';
 
 import 'package:path/path.dart' as p;
 
@@ skipping 53 matching lines @@
     // Remove the children of [dir], as well as [dir] itself if necessary.
     //
     // [partialPath] is the path to [dir], and a prefix of [path]; the remaining
     // components of [path] are in [parts].
     recurse(dir, partialPath) {
       if (parts.length > 1) {
         // If there's more than one component left in [path], recurse down to
         // 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);
         // After removing this entry's children, if it has no more children and
         // it's not in the set in its own right, remove it as well.
         if (entry.isEmpty && !_paths.contains(partialPath)) dir.remove(part);
         return paths;
       }
 
       // If there's only one component left in [path], we should remove it.
@@ skipping 74 matching lines @@
     // 14980).
     if (!p.isRelative(relative) || parts.first == '..' || parts.first == '.') {
       throw new ArgumentError('Path "$path" is not inside "$root".');
     }
     return p.join(root, relative);
   }
 
   /// Returns the segments of [path] beneath [root].
   List<String> _split(String path) => p.split(p.relative(path, from: root));
 }