Web app gets sanitized

[shaun-man]

Describe the bug
Web app gets sanitized when running

To reproduce
Steps to reproduce the behavior:

1. Using Android Studio Flutter dev environment, create project using the example Model Viewer Plus app
2. Add button element (see below) via innerModelViewerHtml, add any JS via relatedJs
3. Build for Chrome or Edge
4. Look in the Run tab, messages appear saying "Removing disallowed element ..." and "Removing disallowed attribute ..."
5. In the debug browser, the button appears top left instead of on the model
6. Use DevTools to see that the script element was removed, and attributes of the button have been removed

Expected behavior
The web app includes the button element and the script element exactly as specified in main.dart

More details
Here is the button element that you can use for testing:
'''

Hole 1

'''

[Foldblade]

Sorry for that. Please try #45.