index.html

main.dart

// SO 29434359
import 'dart:html';
import 'dart:async' show Future;

main() async {
  var exists = await urlExists('https://developer.mozilla.org/en-US/docs/Web/API/HTMLTextAreaElement/autofocus');
  print(exists);
  exists = await urlExists("https://developer.mozilla.org/en-US/docs/Web/API/HTMLTextAreaElement");
  print(exists);
}

Future<bool> urlExists(String url) async {
   try {
    await HttpRequest.getString(url);
    return true;
  } catch(error) {
    return false;
  }  // examine response for errors
}

style.css

p{
  
}