add async to canLaunch

packages/url-launcher/lib/url_launcher.dart

@@ -23,10 +23,10 @@ Future<Null> launch(String urlString) {
 
 /// Checks whether the specified URL can be handled by some app installed on the
 /// device.
-Future<bool> canLaunch(String urlString) {
+Future<bool> canLaunch(String urlString) async {
   if (urlString == null)
     return false;
-  return _channel.invokeMethod(
+  return await _channel.invokeMethod(
     'canLaunch',
     urlString,
   );