DRY out a copy-and-paste comment.
Twaek comments.
Realm constructor: instead of defaulting options to an empty object inheriting from %ObjectPrototype%, default to `Object.create(null)`. (Also remove curly braces around some one-liners.)
Use GetRealmInternalData consistently, saving a bunch of code.
More fill-paragraph.
Spec text informally describing loader.[[Global]] is good enough; remove the XXX comment on it.
Consistently say "this Loader", not "this Loader object", and don't spam type-checking steps everywhere that are redundant with what thisLoader() already does.
DRY comment about leaking the use of promises.
Tidy up explanatory spec text about the `instantiate` hook.
Tweak a comment.