fix: Stop passing nullable values to Future<nn-type>.value

[srawlins]

Description

This is cleanup work required to start enforcing this with static analysis, as per dart-lang/sdk#53253.

Real quick this issue is that this code is unsafe:

void f(Completer<int> c, int? i) {
  Future<int>.value(i); // Ouch!
  c.complete(i);        // Ouch!
}

The fixes here are generally to make local test variables late instead of nullable.

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Breaking Change

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• Yes, this is a breaking change.
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[russellwheatley]

Thank you for raising this PR! We have already removed those nullable value checks from those files 😄. Closing out.