add test that repros issue #47338
add test that repros issue #47338

This currently fails for the CFE with:

static error failures:
- Unexpected error at line 137, column 14: A value of type 'int?' can't be assigned to a variable of type 'int' because 'int?' is nullable and 'int' isn't.
- Unexpected error at line 137, column 25: Operand of null-aware operation '!' has type 'int' which excludes null.

Bug: https://github.com/dart-lang/sdk/issues/47338
Change-Id: Ia55e19a0bf296a8427f6510d6ddda5016acdecea
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/215101
Reviewed-by: Johnni Winther <0f64ef784d57ffb064b634040f9eeba0d33d0be4@google.com>
Commit-Queue: Jake Macdonald <28dccc213a1e50b0de348b9cb82c0229ef6b65f0@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk