When I'm running it in Dartium it does cause an exception too.
In your jeu.html you have
<input id="nextPageButton" type="button"/>
And in your main.dart you have:
ButtonElement b = querySelector("#nextPageButton");
And so the error message in Dart is:
Exception: type 'InputElement' is not a subtype of type 'ButtonElement' of 'b'.
Either use a button
-tag in your html or an InputElement
in your main.dart
to fix this issue.