Standard javascript now has Object.assign() which is very similar to
Params.parse(), except that the latter by default disallows "extra"
parameters. We can still leverage the standard API by simply
implementing the error check, and then call out to Object.assign()
for the actual parameter merging.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/612
js2-mode is no longer developed and we recommend js-mode these days,
so switch the modelines to specify that, and make them consistent
across all files.
https://bugzilla.gnome.org/show_bug.cgi?id=660358