* improve gulp generate-runtime-helpers error message
If you had some garbage in packages/babel-runtime-helpers/src/helpers/,
gulp generate-runtime-helpers would blow up with an unhelpful message:
TypeError: Cannot read property 'groups' of null
* ignore files starting with a dot when generating runtime helpers
HELPERS_FOLDER.pathname may be URL-encoded, if the working tree's
absolute path contains spaces, some reserved characters, or anything
beyond ASCII.
readFile accepts a plain String or URL for the path, but does not
automatically decode URL-encoded Strings.
* Allow putting helpers in individual files
* Lint
* Also move `objectSpread2` (to show an helper with deps)
* Update fixture and package.json
* fixture