remove seemingly unused precompiled files concept in `FlutterPlatform`

[andrewkolos]

WIP. This is an experiment.

While trying to learn what FlutterPlatform is, I noticed some constructor parameters that are seemingly unused (maybe they are in g3?). This PR tries to remove them.

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, including Features we expect every widget to implement.
• [ ] I signed the CLA.
• [ ] I listed at least one issue that this PR fixes in the description above.
• [ ] I updated/added relevant documentation (doc comments with ///).
• [ ] I added new tests to check the change I am making, or this PR is test-exempt.
• [ ] I followed the breaking change policy and added Data Driven Fixes where supported.
• [ ] All existing and new tests are passing.

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