It's a good language though, and if you have experience with C#/Swift/Kotlin/Java etc...you'll be productive from day 1...there's not really much to learn, I wouldn't call it a problem.
https://www.dartlang.org/faq#q-why-are-generics-covariant
Of course this is from the company that brought us interface{} and people seem to be OK with that...
Why are covariant generics bad? (also just a link is fine)
As an example, here's approximately how the Function trait is defined in Scala. The square brackets indicate type parameters (like angle brackets in Java), the - indicates covariance, and the + indicates contravariance.
trait Function1[-T, +R] {
def apply(arg1: T): R
}