How to specify a default value of a function parameter?

if you want to initialize a Function parameter that is also a field of your class I suggest:

class MyClass{
  Function myFunc;
  MyClass({this.myFunc = _myDefaultFunc}){...}
  static _myDefaultFunc(){...}
}

Or more suitable:

typedef SpecialFunction = ReturnType Function(
                              FirstParameterType firstParameter, 
                              SecondParameterType secondParameter);

class MyClass{
  SpecialFunction myFunc;
  MyClass({this.myFunc = _myDefaultFunc}){...}
  static ReturnType _myDefaultFunc(FirstParameterType firstParameter, 
                                   SecondParameterType secondParameter){...}
}

You can define the default function as private method :

_defaultTransform(Something v) => v;
void _doSomething(List<Something> numbers, 
                  [transform(Something element) = _defaultTransform]) {...}

Or check argument like this :

void _doSomething(List<Something> numbers, [transform(Something element)]) {
  if (!?transform) transform = (v) => v;
  ...
}

Or like Ladicek suggests :

void _doSomething(List<Something> numbers, [transform(Something element)]) {
 transform ??= (v) => v;
  ...
}