Why 'await' requires 'async' in function definition

I'm currently learning Dart, but this is also applicable to what's going on in the JavaScript world right now, and it seems like C# also uses the same pattern.

In Dart, any function that uses await must itself be labeled asynchronous through async as follows:

import "dart:html";

main() async {
  var context = querySelector("canvas").context2D;
  var running = true;

  while (running) {
    var time = await window.animationFrame;
    ...
  }
}

This does not make sense to me. If a function is waiting on an asynchronous function to complete, is it not then considered blocking? Why do JS and Dart require it to be labeled asynchronous? Would it not be the opposite?

To me it would make far more sense if the calling function must use the async keyword if it calls any function that also includes it in its definition. In this pattern, await would be used to convert asynchronous functions to synchronous ones.

This way would also avoid duplicate functions, since right now, libraries seem to always have func() and funcSync() or funcAsync().