fix a strong mode error

pkgs/markdown/example/app.dart

diff --git a/pkgs/markdown/example/app.dart b/pkgs/markdown/example/app.dart
index 2c9a8be..ccb0de9 100644
--- a/pkgs/markdown/example/app.dart
+++ b/pkgs/markdown/example/app.dart
@@ -83,7 +83,7 @@
 }
 
 void _switchFlavor(Event e) {
-  var target = e.currentTarget;
+  var target = e.currentTarget as HtmlElement;
   if (!target.attributes.containsKey('checked')) {
     if (basicRadio != target) {
       basicRadio.attributes.remove('checked');