nnbd: function?.call() must be null-checked

[sortie]

The analyzer doesn't appear to correctly handle a null checked call on a Function? in the following program:

Function? foo;

main() {
  foo?.call();
}

It produces the following analyzer errors:

Analyzing test.dart...
  error • The expression is nullable and must be null-checked before it can be used. • test.dart:3:3 • unchecked_use_of_nullable_value
1 error found.

These calls are supposed to work after talking to @lrhn and @eernstg.

The issue occurs for me at 43b69d4.

This issue blocks getting dart:_http clean of analyzer errors in NNBD mode as it has two such calls.

[vsmenon]

@stereotype441 tentatively bumping priority on this, as it's the only issue left keeping the ddc sdk from being analyzer clean:

https://github.com/dart-lang/sdk/blob/master/pkg/dev_compiler/tool/dartdevc_nnbd_sdk_error_golden.txt

If this will take a while, we could probably workaround in dart:_http ( @sortie ?). That would unblock unforking / legacy, but we'll likely hit this shortly after that.

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/135185