cleanup to bin/markdown.dart

pkgs/markdown/bin/markdown.dart

diff --git a/pkgs/markdown/bin/markdown.dart b/pkgs/markdown/bin/markdown.dart
index 0df8040..fb6a460 100644
--- a/pkgs/markdown/bin/markdown.dart
+++ b/pkgs/markdown/bin/markdown.dart
@@ -6,30 +6,14 @@
 Future main(List<String> args) async {
   if (args.length > 1) {
     print('Usage: markdown.dart [file]');
-    exit(1);
+    exitCode = 1;
+    return;
   }
 
   if (args.length == 1) {
-    var arg = args.first;
-    if (arg == '--help') usage();
-    if (arg == '--version') {
-      print(version);
-      exit(0);
-    }
-    // Read argument as a file path.
-    print(markdownToHtml(new File(args[0]).readAsStringSync()));
-    exit(0);
-  }
-
-  // Read from stdin.
-  var buffer = new StringBuffer();
-  var line;
-  while ((line = stdin.readLineSync()) != null) buffer.writeln(line);
-  print(markdownToHtml(buffer.toString()));
-}
-
-void usage() {
-  print('''Usage:
+    switch (args.first) {
+      case '--help':
+        print('''Usage:
     markdown [markdown file]
         Convert [markdown-file] from Markdown to HTML. If no file is passed on
         the commandline, then the Markdown source is read from STDIN.
@@ -40,5 +24,24 @@
     markdown --help
         Print this help text.
   ''');
-  exit(0);
+        return;
+
+      case '--version':
+        print(version);
+        return;
+
+      default:
+        // Read argument as a file path.
+        print(markdownToHtml(new File(args[0]).readAsStringSync()));
+        return;
+    }
+  }
+
+  // Read from stdin.
+  var buffer = new StringBuffer();
+  String line;
+  while ((line = stdin.readLineSync()) != null) {
+    buffer.writeln(line);
+  }
+  print(markdownToHtml(buffer.toString()));
 }