Front end produces bogus "Expected ','" message for type mismatch in spread collection

[stereotype441]

The following code:

f(Object? objectQuestion) {
  return {...<int>{}, ...objectQuestion};
}

main() {}

produces these errors from the front end:

tests/language/spread_collections/issue_qqq_error_test.dart:6:26: Error: Unexpected type 'Object?' of a map spread entry.  Expected 'dynamic' or a Map.
 - 'Object' is from 'dart:core'.
  return {...<int>{}, ...objectQuestion};
                         ^
tests/language/spread_collections/issue_qqq_error_test.dart:6:26: Error: Expected ',' before this.
  return {...<int>{}, ...objectQuestion};
                         ^

The first message is ok (technically this is a set spread entry, not a map spread entry, but let's not fret about that right now). The second message is totally bogus.

This bug is blocking my work on "why not promoted" functionality (#44898), and I think I see how to fix it, so I'm going to go ahead and work on it now.