Callable extensions can be invoked but cannot be passed as parameters.

Callable extensions can be invoked but not passed as parameters. In the following example, I create an extension on `List` that implements the callable API. This allows `List`'s to be invoked like functions which works as expected in `exampleMain()`. However, even with this extension, `List` type variables cannot be passed as `List Function(int)` type parameters. See the last line of `exampleMain()`.
```
extension on List {
List call(int maxLength) {
return this;
}
}

void someFunc(List Function(int) getList) {
// Do something
}

void exampleMain(List Function(int) getList) {
List someList = [];
List theSameList = someList(50); // Okay
someFunc(someList); // 'List' can't be assigned ... to 'List Function(int)'
}
```
It might break something that I'm not thinking of, but in the above example it feels like `someList` should be able to be passed to `someFunc()`.

Start with the example in the issue and compare invoking someList with passing it to someFunc. Review Dart's callable-extension and function-type rules to determine whether this behavior is intended and what language change would be required; done means the behavior has a settled specification or implementation direction.

Written by the indexing model from the issue text.