automate building and committing of compiled code that passes tests

Even though `gulp` now does a rebuild the first time it's run, it's still possible to commit code in js/ that's out of sync with build/. For example, if you make a really quick change to a source file and forget to run `gulp` then commit that change things will be out of sync.

This will probably take some doing to implement so in the meantime one of the things all code reviewers should check for is whether or not build files were commit or not. If there are any changes to js/ then there should be changes to at least one file in build/.

Start by examining the existing gulp rebuild behavior and how source files in js/ produce compiled files in build/. Determine how the build-and-commit workflow should run while preserving passing tests. Done means changes to js/ cannot leave build/ out of sync and the resulting compiled files are committed automatically.

Written by the indexing model from the issue text.