always_specify_types

From the flutter style guide:

DO specify type annotations.

Avoid var when specifying that a type is unknown and short-hands that elide type annotations. Use dynamic if you are being explicit that the type is unknown. Use Object if you are being explicit that you want an object that implements == and hashCode.

GOOD:

int foo = 10;
final Bar bar = Bar();
String baz = 'hello';
const int quux = 20;

BAD:

var foo = 10;
final bar = Bar();
const quux = 20;

NOTE: Using the the @optionalTypeArgs annotation in the meta package, API authors can special-case type variables whose type needs to by dynamic but whose declaration should be treated as optional. For example, suppose you have a Key object whose type parameter you'd like to treat as optional. Using the @optionalTypeArgs would look like this:

import 'package:meta/meta.dart';

@optionalTypeArgs
class Key<T> {
 ...
}

main() {
  Key s = Key(); // OK!
}

Incompatible with: avoid_types_on_closure_parameters, omit_local_variable_types.