This patch unifies node6 transpilation:
- instead of generating multiple top-level directories, prefixed with
`node6-`, all transpiled code gets placed under single `node6/` folder
- transpilation doesn't change require paths of transpiled modules any
more
This patch:
- renames ChromiumDownloader into just Downloader (this is in
preparation for different products download)
- moves Downloader from utils/ to lib/. This unifies all of the
production-critical code in the lib/.
Drive-by: make Downloader a regular class.
This patch introduces check_availability.js utility which looks for
available chromium binaries for different revisions and platforms.
This patch also re-factors the chromium downloader scripts so that
it can operate different platforms.
