remove _child from ASLayoutSpec.mm. Store all children in the children array.

AsyncDisplayKit/Layout/ASBackgroundLayoutSpec.mm

@@ -68,9 +68,4 @@ static NSString * const kBackgroundChildKey = @"kBackgroundChildKey";
   return [super childForIdentifier:kBackgroundChildKey];
 }
 
-- (void)setChildren:(NSArray *)children
-{
-  ASDisplayNodeAssert(NO, @"not supported by this layout spec");
-}
-
 @end

AsyncDisplayKit/Layout/ASInsetLayoutSpec.mm

@@ -115,9 +115,4 @@ static CGFloat centerInset(CGFloat outer, CGFloat inner)
   return [ASLayout layoutWithLayoutableObject:self size:computedSize sublayouts:@[sublayout]];
 }
 
-- (void)setChildren:(NSArray *)children
-{
-  ASDisplayNodeAssert(NO, @"not supported by this layout spec");
-}
-
 @end

AsyncDisplayKit/Layout/ASLayoutSpec.mm

@@ -26,7 +26,6 @@
   ASEnvironmentState _environmentState;
   ASDN::RecursiveMutex _propertyLock;
   
-  id<ASLayoutable> _child;
   NSArray *_children;
   NSMutableDictionary *_childrenWithIdentifier;
 }
@@ -118,7 +117,7 @@
   ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
   
   id<ASLayoutable> finalLayoutable = [self layoutableToAddFromLayoutable:child];
-  _child = finalLayoutable;
+  _children = @[finalLayoutable];
   [self propagateUpLayoutable:finalLayoutable];
 }
 
@@ -128,6 +127,7 @@
   
   id<ASLayoutable> finalLayoutable = [self layoutableToAddFromLayoutable:child];
   self.childrenWithIdentifier[identifier] = finalLayoutable;
+  self.children = [self.children arrayByAddingObject:finalLayoutable];
   
   // TODO: Should we propagate up the layoutable at it could happen that multiple children will propagated up their
   //       layout options and one child will overwrite values from another child
@@ -156,7 +156,7 @@
 
 - (id<ASLayoutable>)child
 {
-  return _child;
+  return [_children firstObject];
 }
 
 - (NSArray *)children

AsyncDisplayKit/Layout/ASOverlayLayoutSpec.mm

@@ -61,11 +61,6 @@ static NSString * const kOverlayChildKey = @"kOverlayChildKey";
   return [ASLayout layoutWithLayoutableObject:self size:contentsLayout.size sublayouts:sublayouts];
 }
 
-- (void)setChildren:(NSArray *)children
-{
-  ASDisplayNodeAssert(NO, @"not supported by this layout spec");
-}
-
 @end
 
 @implementation ASOverlayLayoutSpec (Debugging)

AsyncDisplayKit/Layout/ASRatioLayoutSpec.mm

@@ -75,11 +75,6 @@
   return [ASLayout layoutWithLayoutableObject:self size:sublayout.size sublayouts:@[sublayout]];
 }
 
-- (void)setChildren:(NSArray *)children
-{
-  ASDisplayNodeAssert(NO, @"not supported by this layout spec");
-}
-
 @end
 
 @implementation ASRatioLayoutSpec (Debugging)

AsyncDisplayKit/Layout/ASRelativeLayoutSpec.mm

@@ -92,11 +92,6 @@
   return [ASLayout layoutWithLayoutableObject:self size:size sublayouts:@[sublayout]];
 }
 
-- (void)setChildren:(NSArray *)children
-{
-  ASDisplayNodeAssert(NO, @"not supported by this layout spec");
-}
-
 - (CGFloat)proportionOfAxisForAxisPosition:(ASRelativeLayoutSpecPosition)position
 {
   if ((position & ASRelativeLayoutSpecPositionCenter) != 0) {

AsyncDisplayKit/Layout/ASStackLayoutSpec.mm

@@ -121,17 +121,6 @@
   _baselineRelativeArrangement = baselineRelativeArrangement;
 }
 
-- (void)setChild:(id<ASLayoutable>)child forIdentifier:(NSString *)identifier
-{
-  ASDisplayNodeAssert(NO, @"ASStackLayoutSpec only supports setChildren");
-}
-
-- (id<ASLayoutable>)childForIdentifier:(NSString *)identifier
-{
-  ASDisplayNodeAssert(NO, @"ASStackLayoutSpec only supports children");
-  return nil;
-}
-
 - (ASLayout *)measureWithSizeRange:(ASSizeRange)constrainedSize
 {
   if (self.children.count == 0) {

AsyncDisplayKit/Layout/ASStaticLayoutSpec.mm

@@ -72,17 +72,6 @@
                                    sublayouts:sublayouts];
 }
 
-- (void)setChild:(id<ASLayoutable>)child forIdentifier:(NSString *)identifier
-{
-  ASDisplayNodeAssert(NO, @"ASStaticLayoutSpec only supports setChildren");
-}
-
-- (id<ASLayoutable>)childForIdentifier:(NSString *)identifier
-{
-  ASDisplayNodeAssert(NO, @"ASStaticLayoutSpec only supports children");
-  return nil;
-}
-
 @end
 
 @implementation ASStaticLayoutSpec (ASEnvironment)