• Add more documentation.
• Make premature binding accesses have better runtime errors.
• Make accesses to instance not required !.
• Rationalize our "ensureInitialized" etc logic.

Fixes #64830
Fixes #78028

Most of these changes were done using dart fix --apply, but they have been squashed into one commit for easier merging.
To see previous reviews of this PR, see
#83843,
#86388,
#86438, and
#89451.

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 feature I am adding, or Hixie said the PR is test-exempt.
• All existing and new tests are passing.