[ASCellNode] implement a .viewController property

AsyncDisplayKit/ASCellNode.h

@@ -116,6 +116,13 @@ typedef NS_ENUM(NSUInteger, ASCellNodeVisibilityEvent) {
  */
 @property (nonatomic, readonly, nullable) NSIndexPath *indexPath;
 
+/**
+ * The backing view controller, or @c nil if the node wasn't initialized with backing view controller
+ * @note This property must be accessed on the main thread.
+ */
+@property (nonatomic, readonly, nullable) UIViewController *viewController;
+
+
 /**
  * The owning node (ASCollectionNode/ASTableNode) of this cell node, or @c nil if this node is
  * not a valid item inside a table node or collection node or if those nodes are nil.

AsyncDisplayKit/ASCellNode.mm

@@ -236,6 +236,12 @@ static NSMutableSet *__cellClassesForVisibilityNotifications = nil; // See +init
   return nil;
 }
 
+- (UIViewController *)viewController
+{
+  ASDisplayNodeAssertMainThread();
+  return _viewController;
+}
+
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wobjc-missing-super-calls"