[dart:js_interop]: Add a way to create a Dart-typed function using the JSFunction constructor.

[ykmnkmi]

For example, instead of writing this:

import 'dart:js_interop';

@JS('Function')
external JSFunction _createFunction2(String first, String second, String body);

final JSFunction _add = _createFunction2('a', 'b', 'return a + b');

int add(int a, int b) {
  var result = _add.callAsFunction(null, a.toJS, b.toJS) as JSNumber;
  return result.toDartInt;
}

void main() {
  print(add(1, 2));
}

write it this way:

import 'dart:js_interop';

@JS('Function')
external JSFunction<int Function(int, int)> _createFunction2(String first, String second, String body);

final JSFunction<int Function(int, int)> add = _createFunction2('a', 'b', 'return a + b');

void main() {
  print(add(1, 2)); // If a direct call is possible.
}

[srujzs]

Similar issue on typing functions that I still want to get to: #54557

The direct call here is really interesting. I'm not sure if we can implement call in such a way that it can capture the parameter and return types and the arity of the function type, but it would be quite neat. It would make JSFunctions very Dart-like in syntax.

[ykmnkmi]

If not, it would be nice to have T function = JSFunction<T>.toDart.