bool container

If 'container' is true, this RenderObject will introduce a new node in the semantics tree. Otherwise, the semantics will be merged with the semantics of any ancestors.

The 'container' flag is implicitly set to true on the immediate semantics-providing descendants of a node where multiple children have semantics or have descendants providing semantics. In other words, the semantics of siblings are not merged. To merge the semantics of an entire subtree, including siblings, you can use a RenderMergeSemantics.

Source 

bool get container => _container;
void container=(bool value)

Source 

set container(bool value) {
  assert(value != null);
  if (container == value)
    return;
  _container = value;
  markNeedsSemanticsUpdate();
}