Try getting a member variable given by the constructor to final. I don't have an exhaustive list of what Dart lacks, but it doesn't take long when coding to start getting frustrated. While syntactically similar, it lacks certain features that make it a pain to use. The problem is, I find Dart to be worse than Java. At first glance you think it's basically Java, and that by switching to it from something like Kotlin, you'll just be downgrading to Java again. I think when you actually sit down and try to use Dart it's clear why people aren't flocking to it. You can see this clearly in the languages ecosystem, as it has relatively few OSS libraries that are actively maintained by non-Google parties. That being said, I'm not a fan of Dart the language, and while there are a few super outspoken developers for Dart, the majority of the developer community has stayed away from the language, and it hasn't really found much usage outside of Google.

Since Google owns Dart, they could customize Dart to meet their requirements. They wanted to compete with React-native with instant deploys for development, which means an interpreted language, but they also can't use a JIT on iOS devices, so they needed a language that could also be ahead-of-time compiled. Kotlin is the clear choice now, but Kotlin/native didn't exist when the Flutter team was switching off of JavaScript.

Flutter with kotlin