Macros: Allow extendsType to extend a class generated by the macro

[tylandercasper]

I'm trying to make a macro that generates getters and setters that the user can easily initialize in the constructor.

The only way to do that, as far as I am aware, is to do something like:

class GeneratedParentClass {
  int? _x;
  int? get x => _x;
  set x(int? val) {
    _x = val;
    //Do something interesting
  }

  GeneratedParentClass(int? x) : _x = x {
    this.x = x;
  }
}

class BaseClass extends GeneratedParentClass {
  BaseClass(super.x);
}

in the build_runner system, this works pretty well, but in Macros, this appears to be impossible.

unlike appendMixins or appendInterefaces, extendsType requires a NamedTypeAnnotationCode, which requires an identifier.
I tried to make a FakeIdentifier which extended Identifier, and took a string, but unfortunately that crashed the inspector.

I think the ability to use created classes for extension would make macros significantly more robust and intuitive, since it would allow for a couple things:

1. overrriding methods created by the macro.
   For example, if a user wants to modify the result of a generated toJson() call, they could simply do:

  @override
  toJson() {
    var json = super.toJson();
    //modify json as desired
  }

2. Guarantee the ability for the macro to initialize code as needed.

3. allow generated variables to be set in constructors. (Which is very important for most widget based code)