Using the `Completer` API in Dart to make a JavaScript API more Dart idiomatic

fetch_contacts_v2.dart

@JS()
library fetch_contacts_interop;

import 'dart:async';

import 'package:func/func.dart';
import 'package:js/js.dart';

// Natively calls "fetch_contacts", but we can use typed Dart code while developing!
@JS('fetch_contacts')
external void _jsFetchContacts(String prefix, VoidFunc1<List<String>> callback);

// Here is our new function!
Future<List<String>> fetchContacts(String prefix) {
  final completer = new Completer<List<String>>();
  _jsFetchContacts(prefix, completer.complete);
  return completer.future;
}

// Example use, including async/await!
main() async {
  print('Contacts: ${await fetchContacts('A')}');
}