Fix #155 - allows users to omit the top-level script tag if it is basically
empty. This makes it possible to treat any HTML file as an entrypoint.

If a file doesn't have a script tag, we assume it to be:
   <script type='application/dart'>main() {}</script>

We also provide a --components_only flag, that can be used to avoid generating
the entrypoint and bootstrap logic, and generate only the components defined in
a file.

Review URL: https://chromiumcodereview.appspot.com//11699003