chore: update to 6to5 v2.2

This new version uses core-js (zloirock/core-js), which is smaller than
ES6-shim (they said, significantly smaller).

The `commonInterop` module formatter has been merged into `common`. It has
some problems about default exporting (6to5/6to5#356). That's why some specs
have also been fixed in this commit.
Merge pull request #49 from bemusic/6to5-2.2

chore: update to 6to5 v2.2