Add support for reverse direction to yoga layouts

Source/Layout/ASStackLayoutDefines.h

@@ -15,6 +15,12 @@ typedef NS_ENUM(NSUInteger, ASStackLayoutDirection) {
   ASStackLayoutDirectionVertical,
   /** Children are stacked horizontally */
   ASStackLayoutDirectionHorizontal,
+#if YOGA
+  /** Children are stacked vertically, but in reverse. Only used by Yoga spec. */
+  ASStackLayoutDirectionVerticalReverse,
+  /** Children are stacked horizontally, but in reverse. Only used by Yoga spec. */
+  ASStackLayoutDirectionHorizontalReverse,
+#endif
 };
 
 /** If no children are flexible, how should this spec justify its children in the available space? */

Source/Layout/ASStackLayoutSpec.mm

@@ -193,6 +193,13 @@ - (void)resolveVerticalAlignment
     case ASStackLayoutDirectionHorizontal:
       [result insertObject:@{ (id)kCFNull: @"horizontal" } atIndex:0];
       break;
+#if YOGA
+    case ASStackLayoutDirectionVerticalReverse:
+    case ASStackLayoutDirectionHorizontalReverse:
+      // Currently not handled.
+      ASDisplayNodeFailAssert(@"Reverse directions not implemented.");
+      break;
+#endif
   }
 
   return result;

Source/Layout/ASYogaUtilities.mm

@@ -96,7 +96,16 @@ YGAlign yogaAlignSelf(ASStackLayoutAlignSelf alignSelf)
 
 YGFlexDirection yogaFlexDirection(ASStackLayoutDirection direction)
 {
-  return direction == ASStackLayoutDirectionVertical ? YGFlexDirectionColumn : YGFlexDirectionRow;
+  switch (direction) {
+    case ASStackLayoutDirectionVertical:
+      return YGFlexDirectionColumn;
+    case ASStackLayoutDirectionVerticalReverse:
+      return YGFlexDirectionColumnReverse;
+    case ASStackLayoutDirectionHorizontal:
+      return YGFlexDirectionRow;
+    case ASStackLayoutDirectionHorizontalReverse:
+      return YGFlexDirectionRowReverse;
+  }
 }
 
 float yogaFloatForCGFloat(CGFloat value)