Remove globals_null_migrated.dart, move into globals.dart

[jmagman]

Not sure how this happened, but I lost a few imports and actually deleting globals_null_migrated.dart in #92861

Part of #71511

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].
• All existing and new tests are passing.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat.

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[christopherfujino]

LGTM

[fluttergithubbot]

This pull request is not suitable for automatic merging in its current state.

• The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label.

[jmagman]

Google testing is mad that #92861 hasn't yet rolled.