Merging PR #8339 on behalf of @hansl to branch presubmit-hansl-pr-8339.

for the record, we need to instead prevent using 1.9 features in our code by running all .d.ts files through TS typechecker in the typescript_next build.

This is just a quick hack to unblock the release.

@chuckjaz is thinking of using a TS language service to do downleveling, but I think we only need that for features we can't avoid using (eg. readonly)

lgtm! thanks

Merging PR #8339 on behalf of @mprobst to branch presubmit-mprobst-pr-8339.

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}