Add usage to explainer build script

example/explainer/build_examples.dart

 #!/usr/bin/env dart
 // Copyright (c) 2012, 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.
 
 /**
  * Script to compile each dart web component examples and copy the
  * generated code to an output directory.
  */
 library build_examples;
 
 import 'dart:io';
 import 'package:args/args.dart';
 import 'package:web_ui/dwc.dart' as dwc;
 
 main() {
   var argParser = new ArgParser();
+  argParser.addFlag('help', abbr: 'h',
+      help: 'Displayes this help message', defaultsTo:false, negatable: false);

[Jennifer Messerly, 2012/12/12 02:02:03]

nit, missing space in "defaultsTo:false" (and corresponding line wrap). it might
fit like this:

  argParser.addFlag('help', abbr: 'h', help: 'Displayes this help message',
      defaultsTo: false, negatable: false);

[Siggi Cherem (dart-lang), 2012/12/13 18:05:23]

Done.

   argParser.addOption('out', abbr: 'o',
       help: 'output directory for the generated code',
       defaultsTo: 'generated');
   var args = argParser.parse(new Options().arguments);
 
+  if (args['help']) {
+    print('Usage: build_examples.dart [-o outdir] [file1, file2, ...]');
+    print(argParser.getUsage());
+    exit(0);
+  }
+
   var output = args['out'];
   if (args.rest.isEmpty) {
     var dir = new Directory.current();
     listFiles(dir,
         (filename) => filename.endsWith('.html') && !filename.startsWith('_'))
         .then((inputs) {
           buildAll(inputs.map((file) => new Path(file).filename), output);
         });
   } else {
     buildAll(args.rest, output);
@@ skipping 48 matching lines @@
 final String NO_COLOR = '\u001b[0m';
 
 Stopwatch startTime() => new Stopwatch()..start();
 
 void stopTime(Stopwatch watch, String message) {
   watch.stop();
   var duration = watch.elapsedMilliseconds;
   print('$message: $GREEN_COLOR$duration ms$NO_COLOR');
   totalTime.add('$message: $GREEN_COLOR$duration ms$NO_COLOR');
 }