fixes #610, incorrect help output

tool/build_sdk.dart

 #!/usr/bin/env dart
 // Copyright (c) 2016, 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.
 
 /// A straightforward script that builds the SDK.
 ///
 /// This would be easy enough to do in a shell script, as we go through the
 /// command line interface. But being able to build from a Dart library means
 /// we can call this during code coverage to get more realistic numbers.
 
-import 'package:args/command_runner.dart' show CommandRunner;
 import 'package:dev_compiler/src/compiler/command.dart';
 
 main(List<String> arguments) {
   var args = [
-    'compile',
     '--unsafe-force-compile',
     '--no-source-map',
     '--no-emit-metadata'
   ];
   args.addAll(arguments);
   args.addAll([
     'dart:_runtime',
     'dart:_debugger',
     'dart:_foreign_helper',
     'dart:_interceptors',
@@ skipping 15 matching lines @@
     'dart:mirrors',
     'dart:typed_data',
     'dart:indexed_db',
     'dart:html',
     'dart:html_common',
     'dart:svg',
     'dart:web_audio',
     'dart:web_gl',
     'dart:web_sql'
   ]);
-  var runner = new CommandRunner('dartdevc', 'Dart Development Compiler');
-  runner.addCommand(new CompileCommand());
-  return runner.run(args);
+  compile(args);
 }