code: @SuppressWarnings("unchecked")
    public SingleFieldBuilder<MType, BType, IType> clear() {
      message = (MType) (message != null ?
          message.getDefaultInstanceForType() :
          builder.getDefaultInstanceForType());
      if (builder != null) {
        builder.dispose();
        builder = null;
      }
      onChanged();
      return this;
    }
docs: Clears the value of the field.
  
  @return the builder