Example of extension of built values list-builder.

build_extension_sample.dart

abstract class Built<V extends Built<V, B>, B extends Builder<V, B>> {
  V rebuild(void Function(B builder) edit);
}
abstract class Builder<V extends Built<V, B>, B extends Builder<V, B>> {
  V build();
}
abstract class ListBuilder<V> {
 V operator [](int index);
 void operator []=(int index, V element);
}

class Value extends Built<Value, ValueBuilder> {
  Value rebuild(void Function(ValueBuilder builder) edit) {
    var b = ValueBuilder.from(this);
    edit(b);
    return b.build();
  }
}

class ValueBuilder extends Builder<Value, ValueBuilder> {
  final Value _original;
  ValueBuilder.from(Value value) : _original = value;
  Value build() => _original;
}

extension ListBuilderRebuildExtension<V extends Built<V, B>, B extends Builder<V, B>> 
    on ListBuilder<Built<V, B>> {
  void rebuildAt(int index, void Function(B) edit) {
    this[index] = this[index].rebuild(edit);
  }
}

class ListBuilderImpl<V> implements ListBuilder<V> {
  List<V> _base = []..length = 10;
  int get length => _base.length;
  V operator [](int index) => _base[index];
  void operator []=(int index, V element) {
    _base[index] = element;
  }
}

void main() {
  ListBuilder<Value> b = ListBuilderImpl();
  b[0] = Value();
  b.rebuildAt(0, (b) {
    var argumentType = staticType(b);
    print("$argumentType: ${b is ValueBuilder}");
    return;
  });
}

Type staticType<T>(T value) => T;