Add usage to explainer build script

example/explainer/build_examples.dart

Index: example/explainer/build_examples.dart
diff --git a/example/explainer/build_examples.dart b/example/explainer/build_examples.dart
index 57b8f6aa3ba471f71fd2d37869e5a9259a9e49d3..3cf406f79559893c47fe875bc93cd257bf0e58ed 100755
--- a/example/explainer/build_examples.dart
+++ b/example/explainer/build_examples.dart
@@ -15,11 +15,19 @@ 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();