Add button not working if run flutter_view example on the web/chrome

I’m very new to code anything with Flutter. I just tried to run flutter_view example on Chrome after made it build/run by adding index.html under web folder. It build and visually run fine, but when I click on the + button it doesn’t do anything. I don’t see any error generated anywhere. It also doesn’t hit the break-point on the function. Please advice.