* Remove dependency on constants.ts
* Don't export LICENSE to WPT export
Our license is the same as WPT's license. Excluding this license from
the export reduces the apparent complexity of WPT's licensing (by
removing a redundant extra license file).
Update CONTRIBUTING.md to explain this.
* Use Babel instead of copy to generate out-wpt/
This lets us use different Babel options for out-wpt/.
Also tweak Grunt targets.
* revise
* retainLines, mkdir in gen_*
