prefer_function_declarations_over_variables

Use a function declaration to bind a function to a name.

From Effective Dart:

DO use a function declaration to bind a function to a name.

As Dart allows local function declarations, it is a good practice to use them in the place of function literals.

BAD:

void main() {
  var localFunction = () {
    ...
  };
}

GOOD:

void main() {
  localFunction() {
    ...
  }
}

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

linter:
  rules:
    - prefer_function_declarations_over_variables

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

linter:
  rules:
    prefer_function_declarations_over_variables: true