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
update changelog to include notes about 0.2.11 and upcoming changes
Updates to the web_ui package to use lib v2

Review URL: https://chromiumcodereview.appspot.com//11855002
Fix #312 - use the correct import location in the bootstrap code.

Review URL: https://chromiumcodereview.appspot.com//11918002
Add a warning about using a component that isn't referenced.
Warning was only being issued for is="", now it is for x- tags too. Also added tests for new and existing warnings.

Review URL: https://chromiumcodereview.appspot.com//12016007