Add isDirty property and make markDirty less crashy/more performant.

Summary: For some internal Instagram diffs, and some external ones (auto-detection of dirty nodes). We need to give the dirty properties a little love. This adds a getter to find out if your node is dirty, and makes sure that the leaf node is properly setup before it is marked dirty to prevent a crash.

Reviewed By: emilsjolander

Differential Revision: D4549241

fbshipit-source-id: 36eda6fdb4ea7f968d126aab6da67896f4a01d40

YogaKit/Source/YGLayout.h

@@ -111,6 +111,12 @@
  */
 @property (nonatomic, readonly, assign) BOOL isLeaf;
 
+/**
+ Return's a BOOL indicating if a view is dirty. When a node is dirty
+ it usually indicates that it will be remeasured on the next layout pass.
+ */
+@property (nonatomic, readonly, assign) BOOL isDirty;
+
 /**
  Mark that a view's layout needs to be recalculated. Only works for leaf views.
  */