Split `run_config` into `run_config download` and `run_config generate`.

bin/generate.dart

 #!/usr/bin/env dart
 
 // Copyright (c) 2015, 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.
 
 import "dart:io";
 
 import "package:args/args.dart";
 import "package:googleapis_generator_dependency/googleapis_generator.dart";
 
 ArgParser downloadCommandArgParser() {
   return new ArgParser()
       ..addOption('output-dir',
                   abbr: 'o',
                   help: 'Output directory of discovery documents.',
                   defaultsTo: 'googleapis-discovery-documents');
 }
 
 ArgParser runConfigCommandArgParser() {
   return new ArgParser()
+      ..addCommand('download')
+      ..addCommand('generate')
       ..addOption('config-file',
                   help: 'Configuration file describing package generation.',
                   defaultsTo: 'config.yaml');
 }
 
 ArgParser globalArgParser() {
   return new ArgParser()
       ..addCommand('download', downloadCommandArgParser())
       ..addCommand('run_config', runConfigCommandArgParser())
       ..addFlag('help', abbr: 'h', help: 'Displays usage information.');
@@ skipping 41 matching lines @@
   } else if (commandOptions == null ||
              !subCommands.contains(commandOptions.name)) {
     dieWithUsage('Invalid command');
   }
 
   switch (commandOptions.name) {
     case 'download':
       downloadDiscoveryDocuments(commandOptions['output-dir']);
       break;
     case 'run_config':
+      if (commandOptions.command == null ||
+          !['download', 'generate'].contains(commandOptions.command.name)) {
+        dieWithUsage('The `run_config` command has only the two subcommands: '
+                     '`download` and `generate`.');
+      }
+
       var configFile = commandOptions['config-file'];
-      generateFromConfiguration(configFile).then((_) => print('Done!'));
+      switch (commandOptions.command.name) {
+        case 'download':
+          downloadFromConfiguration(configFile).then((_) => print('Done!'));
+          break;
+        case 'generate':
+          generateFromConfiguration(configFile);
+          print('Done');
+          break;
+      }
       break;
   }
 }