In Dart, syntactically nice way to cast dynamic to given type or return null?

Question

I have a dynamic x and I would like to assign x to T s if x is T, and otherwise assign null to s. S

Answer 1

Dart 2 has generic functions which allows

T cast(x) => x is T ? x : null;

dynamic x = something();
String s = cast(x);

you can also use

var /* or final */ s = cast(x);

and get String inferred for s