- fix highliting of extends clause in types
- label constants as constants

Change-Id: Iad1682a5c294dc1999067f4e43feb4aada7e1ced
Reviewed-on: https://chromium-review.googlesource.com/c/1451924
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59338}