Yeah, I agree that dart is an awesome programming language but in future, Flutter must come up with some solution for SEO websites to take the current awesomeness of node + react. Until then, websites that rely on SEO, Flutter may not be a good choice for them. Or every crawler starts rendering JS.
It's a little tricky to have dart gain more popularity in web development compared to React or Node. 50% of web apps are built in react and it's becoming better overtime