[web] Migrate Flutter Web DOM usage to JS static interop - 21.

[joshualitt]

This is CL 21 in a series of CLs to migrate Flutter Web DOM usage to the new JS static interop API.

[skia-gold]

Gold has detected about 5 new digest(s) on patchset 11.
View them at https://flutter-engine-gold.skia.org/cl/github/33349

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[eyebrowsoffire]

A couple really small things, otherwise LGTM!