In the an+b syntax, this continues to allow comments and spaces like so:
` an + b `. It does not allow `a n+b`, or `- an+b` or `+ an+b` (and the
same for the `an-b` form). Similarly, it does not allow `- b` or `+ b`.
Additionally, it *does* allow `+/*comment*/n+b` or `-/*comment*/n+b`,
but not `+ n+b` or `-n+b`. This is specced; in this one case we
parse two tokens but do not allow whitespace in between.
MozReview-Commit-ID: INzFGeMPeK7
f11bf6e617