[linter] lines_longer_than_80_chars and document_ignores shouldn't read files

* document_ignores read each file once
* lines_longer_than_80_chars read each file for every line > 80 chars (!).

This follows the approach in
https://dart-review.googlesource.com/c/sdk/+/401424 where the same
pattern was identified as bad.

Before this CL, if a file had, say, 1000 lines > 80 chars (and having
the lint enabled), the lint would read the file - from disk - 1000
times. A somewhat saving grace is that for open files - which I guess is
where it's normally run a lot - it normally reads from an overlay file
and thus not from disk.

Except if you are editing a part in which case the open part is an
overlay, but the "parent" (which is for whatever reason also linted) is
read from file. So if you have 1000 lines > 80 chars in the "parent"
file, and is typing in a part file you're in for a really bad time.

This CL should reduce the number of reads in the lint to 0.

Change-Id: Iac6c78f50add1bc7a411c0c244a181f5ca2ea25b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/411300
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>