[web] Reland "Migrate Flutter Web DOM usage to JS static interop - 3"

[joshualitt]

This is reland of CL 3 in a series of CLs to migrate Flutter Web DOM usage to the new JS static interop API. The issue with CL 3 is that I incorrectly typed the append method.

Unfortunately, I am not able to repro the crashing issue outside of the framework.

Thankfully, there is test coverage for this on the roll, but not in the engine itself.

[flutter-dashboard]

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[joshualitt]

See https://github.com/flutter/engine/compare/c6b9d5ae708752cc17ac5965002f4c424f56d277..9b56d240cb59206d4e3ef2c3a45b20cc213afe66 for the diff

[joshualitt]

Because the fix was so trivial, I attempted to roll this in with CL #4. Leaving this open temporarily in case we need to roll CL #4 back.