Remove implicit dynamic

[rileyporter]

Updates the Flutter usage of Dart js_util methods to utilize the generic return type.

This pull request is part "(3) update these Flutter instances to state the explicit types" of the chain started with: #29192

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above. pre-emptively fixing an issue, no issue to track
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on a refactor with no semantic changes, no tests to add
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the [CLA].
• All existing and new tests are passing.

[flutter-dashboard]

This pull request executed golden file tests, but it has not been updated in a while (20+ days). Test results from Gold expire after as many days, so this pull request will need to be updated with a fresh commit in order to get results from Gold.

[yjbanov]

I'm not sure when, but this has been done already. Closing.