Avoid passing a nullable value to Future<nn-type>.value or Completer<nn-type>.completer.

[srawlins]

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!
}

This change should be a no-op. Adding this explicit null-assert ensures that any exception is thrown right at the null-assert.

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[annagrin]

LGTM!