Least upper bound bug? Analyzer and CFE infer "Function"

Try this in DartPad:

```dart
Future a(String f) async {}
void b([String f]) {}

void foo(T a, T b) {
print(T);
}

void main() {
foo(a, b);
}
```

I would expect the LUB of `(String) -> Future` and `([String]) -> void` to be `(String) -> void`. Manually providing the type argument `` to `foo` doesn't cause any errors at runtime.

Is this an implementation error or a language issue?

cc @eernstg

Run the DartPad reproduction and inspect the type inferred for foo's generic argument by the analyzer and CFE. Compare that result with the language specification's least-upper-bound rules for the two function types; done means determining whether the behavior is an implementation bug or a language issue and recording the expected result.

Written by the indexing model from the issue text.