Declare method return types DRT-W1040

DO declare method return types.

When declaring a method or function always specify a return type. Declaring return types for functions helps improve your codebase by allowing the analyzer to more adequately check your code for errors that could occur during runtime.

BAD:

main() { }

_bar() => _Foo();

class _Foo {
  _foo() => 42;
}

GOOD:

void main() { }

_Foo _bar() => _Foo();

class _Foo {
  int _foo() => 42;
}

typedef predicate = bool Function(Object o);