Remove some dynamics (mark II)

[Hixie]

...and add in some !s that were implied previously. Dynamic really is just a way to hide danger...

This is a reland of #75820 which was reverted in #76232 due to some incompatibilities with Firebase plugins. The PlatformException class' details property has been changed to dynamic rather than Object? to avoid the issue.

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 listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[Hixie]

cc @goderbauer

[goderbauer]

LGTM

[Hixie]

will land if frob goes green (358040385)