Don't clobber js extension handler when using register helper

We want to ease into adopting es6 gently and opt-in for on a per file basis.  Initially we're trying it out in our test files and so I figured the lowest friction way to do this would be to use a test bootstrap file that uses the register helper to only register 6to5 as the extension handler for .es6 files. Unfortunately `hookExtensions` always gets called in register.  Does this PR represent a sensible approach to disabling that behaviour? (It doesn't break any tests, but I'm very new to the codebase) Or is there a better approach you would suggest?