code: private void addRoboInitMethod(MutableClass mutableClass) {
      MethodNode initMethodNode = new MethodNode(Opcodes.ACC_PROTECTED, ROBO_INIT_METHOD_NAME, "()V", null, null);
      RobolectricGeneratorAdapter generator = new RobolectricGeneratorAdapter(initMethodNode);
      Label alreadyInitialized = new Label();
      generator.loadThis();                                         // this
      generator.getField(mutableClass.classType, ShadowConstants.CLASS_HANDLER_DATA_FIELD_NAME, OBJECT_TYPE);  // contents of __robo_data__
      generator.ifNonNull(alreadyInitialized);
      generator.loadThis();                                         // this
      generator.loadThis();                                         // this, this
      writeCallToInitializing(mutableClass, generator);
      // this, __robo_data__
      generator.putField(mutableClass.classType, ShadowConstants.CLASS_HANDLER_DATA_FIELD_NAME, OBJECT_TYPE);
      generator.mark(alreadyInitialized);
      generator.returnValue();
      mutableClass.addMethod(initMethodNode);
    }
docs: Generates code like this:
  ```java
  protected void $$robo$init() {
  if (__robo_data__ == null) {
  __robo_data__ = RobolectricInternals.initializing(this);
  }
  }
  ```