trailing commas for tree-like code in Dart

tree_code_fmt.dart

// frameworks such as Flutter and UIX encourage users to express their
// UI using tree-like syntaxes.

// unfortunately dartfmt will format this code:
main() {
  var componentTree = new MyButton(
    title: 'Save',
    onClick: () {
      doSomthing();
    },
    icon: new Image(
      src: '/images/save.png',
      width: 10
    )
  );
}

// to this:
main() {
  var componentTree = new MyButton(title: 'Save', onClick: () {
    doSomthing();
  }, icon: new Image(src: '/images/save.png', width: 10));
}

// what if Dart allowed trailing commas in function/method/constructor calls and
// dartfmt treated them as a signal that the closing paren must go on the next line?
main() {
  var componentTree = new MyButton(
    title: 'Save',
    onClick: () {
      doSomthing();
    },
    icon: new Image(
      src: '/images/save.png',
      width: 10,  // <-- trailing comma
    ),  // <-- trailing comma
  );
}

// problem solved?

(I understand that what I'm proposing is incompatible with dartfmt's current goals. I think dartfmt's goals should change.)