Await the next event occurring on an element.

nextEvent.dart

// Use this to await the next event occurring on an element.
/* Usage example:
    await querySelector("#myButton").next("click");
    print("Button clicked!");
*/

import "dart:html";
import "dart:async";

extension on Element {
  Future<Event> next(String name) {
    final completer = Completer<Event>();

    void handler(Event event) {
      removeEventListener(name, handler);
      completer.complete(event);
    }

    addEventListener(name, handler);
    return completer.future;
  }
}

void main() {}

nextEvent.js

// Use this to await the next event occurring on an element.
/* Usage example:
    await document.querySelector("#myButton").next("click");
    alert("Button clicked!");
*/
HTMLElement.prototype.next = function(eventName) {
    return new Promise(function(resolve, reject) {
        var handler = function(event) {
            resolve(event);
            this.removeEventListener(eventName, handler);
        };
        this.addEventListener(eventName, handler);
    }.bind(this));
};