Alternative approach to Babel transpilation

[novemberborn]

I've been experimenting with creating a binary cache (#594) of the babel-core dependency: https://github.com/novemberborn/cached-module-loader. On my machine this leads to a 3x improvement in loading babel-core.

I think this would allow us to do Babel transpilation in the test workers themselves, including transpiling helpers (#720) and creating a faster babel-register without precompilation (#945).

I suspect it'll be easier to swap out the Babel transpiler for a TypeScript one, or make other customizations, if we can load it in the test workers rather than the main process. Performance on older Node.js versions (including v4) may not be as good as it is currently, however (#1051).

Curious to hear people's thought on this approach. Happy to provide more detail.

[vadimdemedes]

I think this would allow us to do Babel transpilation in the test workers themselves

This would be just awesome!