Dart: Put generated .mojom.dart files under mojoms pacakge.

mojo/dart/embedder/tools/dart_embedder_url_mappings.py

 #!/usr/bin/python
 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This script scans a directory tree for any .mojom files and outputs a
 # list of url_mapping command line arguments for embedder-package: imports.
 # These url_mapping arguments can be passed to gen_snapshot.
 
 # an example output line:
-# --url_mapping=embedder-package:mojo/service.mojom.dart, \
+# --url_mapping=dart:_mojom/mojo/service.mojom.dart, \
 # /.../src/out/Debug/gen/dart_embedder_packages/mojo/service.mojom.dart
 
 import argparse
 import os
 import sys
 
 def scan(package_root, directory, mapped_to):
   for dirname, _, filenames in os.walk(directory):
     # Ignore tests.
     if dirname.endswith('tests'):
@@ skipping 22 matching lines @@
   package_root = os.path.abspath(args.package_directory_root)
   packages = args.packages
   for package in packages:
     mapping = package.split(',', 1)
     directory = os.path.abspath(mapping[0])
     mapped_to = mapping[1]
     scan(package_root, directory, mapped_to)
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv[1:]))