Update Layout Examples to ASDK 2.0 (#2551)

* Objective-c project updated to ASDK 2.0

* fix assets error

* [LayoutSpecExamples] Renamed new example project to be easier to type / find.

* [Bulid] Manually create Sample.xcscheme file to hopefully fix complaining bulid :).

* clean up project

* commentary on ASAbsoluteLayoutSpec

* address @maicki's comments

examples/LayoutSpecExamples/Sample/LayoutExampleViewController.m

@@ -0,0 +1,48 @@
+//
+//  LayoutExampleViewController.m
+//  Sample
+//
+//  Copyright (c) 2014-present, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under the BSD-style license found in the
+//  LICENSE file in the root directory of this source tree. An additional grant
+//  of patent rights can be found in the PATENTS file in the same directory.
+//
+
+#import "LayoutExampleViewController.h"
+#import "LayoutExampleNodes.h"
+
+@interface LayoutExampleViewController ()
+@property (nonatomic, strong) LayoutExampleNode *customNode;
+@end
+
+@implementation LayoutExampleViewController
+
+- (instancetype)initWithLayoutExampleClass:(Class)layoutExampleClass
+{
+  NSAssert([layoutExampleClass isSubclassOfClass:[LayoutExampleNode class]], @"Must pass a subclass of LayoutExampleNode.");
+  
+  self = [super initWithNode:[ASDisplayNode new]];
+  
+  if (self) {
+    self.title = @"Layout Example";
+    
+    _customNode = [layoutExampleClass new];
+    [self.node addSubnode:_customNode];
+    
+    BOOL needsOnlyYCentering = [layoutExampleClass isEqual:[HeaderWithRightAndLeftItems class]] ||
+                               [layoutExampleClass isEqual:[FlexibleSeparatorSurroundingContent class]];
+                               
+    self.node.backgroundColor = needsOnlyYCentering ? [UIColor lightGrayColor] : [UIColor whiteColor];
+    
+    __weak __typeof(self) weakself = self;
+    self.node.layoutSpecBlock = ^ASLayoutSpec*(__kindof ASDisplayNode * _Nonnull node, ASSizeRange constrainedSize) {
+      return [ASCenterLayoutSpec centerLayoutSpecWithCenteringOptions:needsOnlyYCentering ? ASCenterLayoutSpecCenteringY : ASCenterLayoutSpecCenteringXY
+                                                        sizingOptions:ASCenterLayoutSpecSizingOptionMinimumXY
+                                                                child:weakself.customNode];
+      };
+  }
+  
+  return self;
+}
+
+@end