At long last, this PR is ready for preliminary review. We are now at a point where gulp dist passes in multi-pass and single-pass mode, and gulp check-types, gulp lint, and gulp presubmit are all green. I'm still manually testing the changes I've made, and working through test failures.

Sending this out to all TLs and those familiar with closure compiler. Feel free to assign directories / files to others on your team. For easy reviewing, I've split off the changes into separate commits as far as possible.

Woohoo! Makes me super happy that we are so close to head!

Good news: Build looks good in multi-pass mode with all tests passing (link) and no visual diffs (link)
Bad news: Single pass build is busted, with several test failures (link) Edit: This is now fixed. See latest comments.

Paging @erwinmombay to see if I'm missing something obvious that's causing the single-pass failures. Manual testing confirms that the single-pass build isn't working.

@kristoferbaxter Thanks for keeping me honest with your comments. I've left responses to each of them, and will let you resolve them when you're satisfied that there's no better alternative.

Now on to dealing with the single-pass errors...

The canary cut for the AMP Conf release has been pushed as the 1904091426070 tag. It is now safe to merge this PR. (/cc @choumx who is release on-duty this week)

Once @aghassemi and @newmuis approve the final changes, I'll do another full round of testing and then merge this PR.

This PR is now ready to merge. For posterity, here is a list of things I've reviewed / verified / tested.

Code review:

• Self-reviewed all 240+ files in this PR to make sure there are no stray changes
• Re-read all 200+ review comments to make sure they were satisfactorily addressed and resolved
• Filed issues for future clean up tasks / feature requests

Local tests:

• Ran gulp check-types to make sure there are no type errors
• Ran gulp dist --fortesting to make sure minified multi-pass build is healthy
  • Ran gulp serve --compiled and manually tested several components in multi-pass mode
• Ran gulp dist --fortesting --single_pass --pseudo_names to make sure minified single-pass build is healthy
  • Ran gulp serve --compiled and manually tested several components in single-pass mode
• Ran gulp test --local-changes --headless to make sure all unit tests affected by this PR are green

Travis tests:

• Checks (presubmit, lint, dep-check, and check-types) are green (link)
• Un-minified build is green (link)
• Visual tests are green with zero diffs due to this PR (link)
• All unit tests are green (link)
• All integration tests are green (link)
• Minified single-pass build and tests are green (link)

AMP Release schedule

• AMP Conference 2019 is next week. We're doing a release on Tuesday, Apr 16 using this week's canary cut, which was based on the 1904091426070 tag.
• This PR was merged after the AMP Conf release tag, so the main AMP Conf release is unaffected by any of these changes.

@rsimha Probably needs to be rolled back :( It updated the third_party code improperly. E.g. third_party/config/config.js cannot do dev().assert.

@dvoytenko Thanks for your comment. We can revert the changes to third_party/, as you've done in #21827. For an explanation of why I (incorrectly) made the change, see #21827 (review).

I'll approve and merge your partial revert ASAP.