Move mojo and mojom from mojo/public/dart to mojo/dart/packages

mojo/dart/packages/mojom/lib/src/commands/gen.dart

 // 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.
 
 library mojom.command.tree;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:args/args.dart';
@@ skipping 31 matching lines @@
 
   _validateArguments() async {
     await validateArguments();
     _dartRoot = new Directory(makeAbsolute(argResults['output']));
     if (!await _dartRoot.exists()) {
       throw new CommandLineError(
           'Specified --output directory $_dartRoot does not exist');
     }
   }
 }