Attempting to extend both an interface (abstract class) & implement overrides of those methods, using extension.

gistfile1.txt

abstract class IDataType
{
  void render();
}

class StringDataType implements IDataType
{
  @override
  void render()
  {
    print("Render something here.");
  }
}

// We know we are in the client build target if the following is imported, which will be in a separate file.

extension IClientDataType on IDataType
{
  void run()
  {
    throw new Exception("This method should be considered abstract."); // Unfortunately, this method is not virtual, it appears, because it is an extension method.
  }
}

extension ClientStringDataType on StringDataType
{
  void run()
  {
    print("The 'override' of run() has been executed.");
  }
}

void main() {
  
  Map<String, String> foo = const {};
  
  IDataType dataType = new StringDataType();
  dataType.run();
 
}