Updating elements

These are the steps for rolling new versions of the paper-elements:

1. Run bower update to grab the new versions of the javacript elements.

2. Check if any new elements were added by looking for new folders following the pattern lib/src/paper-*. For each one of these elements, update the files_to_generate section of paper_elements_config.yaml to make it generate new stubs for that element.

3. Run dart tool/update.dart paper_elements_config.yaml.

4. Update the example/ folder by hand. Find all files with the name lib/src/paper-*/demo.html that were modified and update the corresponding example/paper-*.html file. If a new demo file was added then create a new one and migrate it over.