a.dart

class ClassName {
  factory ClassName(Object complicatedObject) => ClassName.internal(
        complicatedObject,
        // Creation of this object is handled in the constructor
        Object(),
      );

  @visibleForTesting
  ClassName.internal(
    this._complicatedObject,
    this._complicatedObject2,
  );

  // Never every directly create a new object here
  // All objects should be set from 
  // ClassName.internal()
  final Object _complicatedObject;
  final Object _complicatedObject2;
}