code: public Object getField(final FieldDescriptorType descriptor) {
      Object o = fields.get(descriptor);
      if (o instanceof LazyField) {
        return ((LazyField) o).getValue();
      }
      return o;
    }
docs: Useful for implementing
  {@link Message#getField(Descriptors.FieldDescriptor)}.  This method
  returns {@code null} if the field is not set; in this case it is up
  to the caller to fetch the field's default value.