[go_router] Avoids using deprecated DartType.element2.

[chunhtai]

unblock roll

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I listed at least one issue that this PR fixes in the description above.
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[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 (don't just cc him here, he won't see it! He's on Discord!).

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?

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[Hixie]

test-exempt: code refactor with no semantic change

[stuartmorgan-g]

I don't understand what this comment means; downgraded analyze uses the lowest version your pubspec allows. The only way to change what version it gets is to change your pubspec, so I don't see what the TODO is here that can't just be done right now instead (or if it can't, why not, and what the plan is for changing that).

[chunhtai]

ah you are right let me try to bump up the analyzer version

[chunhtai]

you are right, bump the pubspec yaml version

[stuartmorgan-g]

Ugh, the latest analyzer doesn't work with Flutter stable's pinned test. We can ignore the deprecation with a TODO instead.