Print options when given no arguments instead of crashing

pkg/docgen/bin/docgen.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.
 
 import 'dart:io';
 
 import 'package:args/args.dart';
 import 'package:logging/logging.dart';
 
 import '../lib/docgen.dart';
 import 'package:path/path.dart' as path;
 
 List<String> excludedLibraries = [];
 
 /**
  * Analyzes Dart files and generates a representation of included libraries,
  * classes, and members.
  */
 void main(List<String> arguments) {
   var results = _initArgParser().parse(arguments);
 
   var includeSdk = results['parse-sdk'] || results['include-sdk'];
   var scriptDir = path.dirname(Platform.script.toFilePath());
   var introduction = includeSdk ? '' : results['introduction'];
-  docgen(results.rest.map(path.normalize).toList(),
+  var files = results.rest.map(path.normalize).toList();
+  if (files.isEmpty) _printHelpAndExit();
+  docgen(files,
       packageRoot: results['package-root'],
       outputToYaml: !results['json'],
       includePrivate: results['include-private'],
       includeSdk: includeSdk,
       parseSdk: results['parse-sdk'],
       append: results['append'] && new Directory(results['out']).existsSync(),
       introFileName: introduction,
       out: results['out'],
       excludeLibraries: excludedLibraries,
       includeDependentPackages: results['include-dependent-packages']);
 }
 
 /**
+ * Print help if we are passed the help option or invalid arguments.
+ */
+void _printHelpAndExit() {
+  print(_initArgParser().getUsage());
+  print('Usage: dart docgen.dart [OPTIONS] fooDir/barFile');
+  exit(0);
+}
+
+/**
  * Creates parser for docgen command line arguments.
  */
 ArgParser _initArgParser() {
   var parser = new ArgParser();
   parser.addFlag('help', abbr: 'h',
       help: 'Prints help and usage information.',
       negatable: false,
       callback: (help) {
-        if (help) {
-          print(parser.getUsage());
-          print('Usage: dart docgen.dart [OPTIONS] fooDir/barFile');
-          exit(0);
-        }
+        if (help) _printHelpAndExit();
       });
   parser.addFlag('verbose', abbr: 'v',
       help: 'Output more logging information.', negatable: false,
       callback: (verbose) {
         if (verbose) Logger.root.level = Level.FINEST;
       });
   parser.addFlag('json', abbr: 'j',
       help: 'Outputs to JSON. Files are outputted to YAML by default. '
         'If --append is used, it takes the file-format of the previous '
         'run stated in library_list.json ignoring the flag.',
@@ skipping 20 matching lines @@
       help: 'Exclude the library by this name from the documentation',
       allowMultiple: true,
       callback: (libs) => excludedLibraries.addAll(libs));
   parser.addFlag('include-dependent-packages',
       help: 'Assumes we are documenting a single package and are running '
         'in the directory with its pubspec. Includes documentation for all '
         'of its dependent packages.',
       defaultsTo: false, negatable: false);
   return parser;
 }