Fixed clowntoun related to `-[RCTShadowView canHaveSubviews]`

Summary:
Shame on me.
Naming can be hard. We have to use positive logic to avoid this kind of bugs. :(

In the bright future we also have to rename `isYogaLeafNode` to something with positive logic, like `canHaveYogaChildNodes`.
But before we can do this, we have to have solid plan how to unify it with Android.

Reviewed By: mmmulani

Differential Revision: D5780917

fbshipit-source-id: 1ddaaea06f5618b91528cc87f1433a55b5fae4ac

React/Views/RCTShadowView.m

@@ -374,7 +374,7 @@ static void RCTProcessMetaPropsBorder(const YGValue metaProps[META_PROP_COUNT],
 
 - (BOOL)canHaveSubviews
 {
-  return NO;
+  return YES;
 }
 
 - (BOOL)isYogaLeafNode
@@ -415,7 +415,7 @@ static void RCTProcessMetaPropsBorder(const YGValue metaProps[META_PROP_COUNT],
 
 - (void)insertReactSubview:(RCTShadowView *)subview atIndex:(NSInteger)atIndex
 {
-  RCTAssert(!self.canHaveSubviews, @"Attempt to insert subview inside leaf view.");
+  RCTAssert(self.canHaveSubviews, @"Attempt to insert subview inside leaf view.");
 
   [_reactSubviews insertObject:subview atIndex:atIndex];
   if (![self isYogaLeafNode]) {