prefer_void_to_null

Don't use the Null type, unless you are positive that you don't want void.

DON'T use the type Null where void would work.

BAD:

Null f() {}
Future<Null> f() {}
Stream<Null> f() {}
f(Null x) {}

GOOD:

void f() {}
Future<void> f() {}
Stream<void> f() {}
f(void x) {}

Some exceptions include formulating special function types:

Null Function(Null, Null);

and for making empty literals which are safe to pass into read-only locations for any type of map or list:

<Null>[];
<int, Null>{};

To enable the prefer_void_to_null rule, add prefer_void_to_null under linter > rules in your analysis_options.yaml file:

linter:
  rules:
    - prefer_void_to_null

If you're instead using the YAML map syntax to configure linter rules, add prefer_void_to_null: true under linter > rules:

linter:
  rules:
    prefer_void_to_null: true