Flutter has moved flutter for web from technical preview to beta.
You can have the same application built for mobile app to be rendered on web with this support.
I believe you need to switch to beta channel.
As of 1.12, web support is available on the beta channel https://flutter.dev/web
Video: A simple project outlining the steps involved.
Please note when this video was made flutter for web was still in technical preview… Make sure to change the channel to beta.