Fix issues with JS backend syntax translation

[sevanspowell]

• Fixed some issues with the JS backend syntax translation introduced by https://github.com/ghcjs/ghcjs-base/pull/135.
  • Just simple text replace issues I suspect.

I needed to make these changes to get the miso examples which depend on ghcjs-base (xhr, file-reader, canvas2d) working. There may be more issues introduced by #135, but I don't have test cases for those atm.

I will do a more thorough review if you feel I'm on the right track. This is my first time modifying JS FFI code.

I also needed to make some changes to miso itself, which I will be submitting PRS for soon.

Thanks for your work on the Javascript backend! ❤️

[sevanspowell]

Changes to miso: https://github.com/dmjio/miso/pull/740.

[hsyl20]

LGTM, thanks!