Flutter Web has a big problem, you like Flutter but then you get to know that Flutter it is not well optimized and has some problems with SEO.
Check this out! I'll give a few trick to use until, they, if, fix this problem.
For Web Flutter use the canvas tag! For those who don't know, the canvas is a tag used for drawing;
It is a really powerfull tag, it let you achieve goals that otherwise are not possible, like complex animations, new ways of interacting with data etc...
Ways that are not possible with the standard HTML, CSS, JS at all.
So what's the problem? seems good right? No bro! The problem is that the Search Engine has not way to understand how your content is made, since it only understands textual content.
Flutter is a great SDK it saves you a ton of time, so I don't se why you should use it anymore, I mean yes it has this problem, but we can find some workarounds.
Before they solve this problem it may take a lot of time, let me tell you how to.
I would say that when you want to develop for the Web you should one take in consideration the project that you want to create!
If you want to create an article based Website like Priiimo.com or This one Dev.to then you can split your website in 2 parts, the part that will show the articles to serve it with node, php or other ways and the hidden part that needs authorization to do it with Flutter; So all the complex part that, for example, contains the management of users, analytics etc...
What do you think? I'll give you another Idea, for example, you can as before use Flutter to create a sort of CMS that will generate HTML pages.
In this way your contente will be visible by search engines without problems.
You have to be careful when serving content to search engines, one solution that probably you had in your mind was to show a clone content of what's visible on your App in a way or another, BUT....
This MAY be a violation of the of Google's guidelines if you are targeting this one; It is called Cloaking.
Here you have a link that explains more about this thing.