Handle watching a non-existent directory.

pkg/watcher/lib/src/utils.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.utils;
+
+import 'dart:io';
+
+/// Returns `true` if [error] is a [DirectoryException] for a missing directory.
+bool isDirectoryNotFoundException(error) {
+  if (error is! DirectoryException) return false;
+
+  // See dartbug.com/12461 and tests/standalone/io/directory_error_test.dart.
+  var notFoundCode = Platform.operatingSystem == "windows" ? 3 : 2;
+  return error.osError.errorCode == notFoundCode;
+}