Make the babel instance configurable in babel-register

[Janpot]

The version of babel that is bundled with babel-register might not always be the same one as the one I want to register
like in this thread: a dependency (a babel plugin) wants to load modules of the dependant with the babel version of the dependant.

I propose to make it configurable like for instance

require('babel/register')({ babel: babelFromSomewhereElse });

I wouldn't mind making a PR for something like this if you agree.

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[Andarist]

This would require some refactoring, as @babel/register is effectful module. To preserve old API and provide a new option it would have to be imported from other directory (i.e @babel/register/custom).

Our team tries to be focused at the moment on releasing v7, so I don't expect anyone starting to work on this right now. Community contribution would be invaluable to get this implemented sooner.

[Janpot]

@Andarist I might not think of all intricacies involved, but to me it seems like it's sufficient to plumb a babel through the options and select it here instead of using the imported babel:

import babelCore from '@babel/core';

// ...

// inside compile()
const babel = opts.babel || babelCore

Wouldn't that be backwards compatible? I can make a PR for that if the babel team agrees.

[Andarist]

Oh yeah, it might very well be that easy. Was looking quickly at the source code and thought that register is more tied to to babel's instance.

I might not think of all intricacies involved,

Me neither 😅 You can start a PR to get ball rolling faster.