Refactor bootstrap.js and bootstrap_helper.js to be able to deal
with generator chunks. In particular for each chunk, specify:
- The goog.module ID to goog.require() in uncompressed mode.
- The script filename to load in compressed mode.
- Where the chunk's UMD wrapper will save the export object when
loaded as a script.
- What global variable the chunk's export object should be saved in
(if desired).
- Any individual named exports to destructure to global variables.
This allows the bootstrap scripts to be slightly simpler while
also being more flexible.
This change had already been made (by PR #6475 amongst other) to
most of the tests, but there were a couple of stragglers that
still mentioned messages.js.
Fixes#6824.
