Add <Modal /> component

Summary:
Create Modal component that can be used to present content modally.

React/Views/RCTModalHostView.m

@@ -0,0 +1,82 @@
+/**
+ * Copyright (c) 2015-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 "RCTModalHostView.h"
+
+#import "RCTAssert.h"
+#import "RCTBridge.h"
+#import "RCTModalHostViewController.h"
+#import "RCTTouchHandler.h"
+#import "RCTUIManager.h"
+#import "UIView+React.h"
+
+@implementation RCTModalHostView
+{
+  RCTBridge *_bridge;
+  BOOL _hasModalView;
+  RCTModalHostViewController *_modalViewController;
+  RCTTouchHandler *_touchHandler;
+}
+
+RCT_NOT_IMPLEMENTED(-initWithFrame:(CGRect)frame)
+
+- (instancetype)initWithBridge:(RCTBridge *)bridge
+{
+  if ((self = [super initWithFrame:CGRectZero])) {
+    _bridge = bridge;
+    _modalViewController = [[RCTModalHostViewController alloc] init];
+    _touchHandler = [[RCTTouchHandler alloc] initWithBridge:bridge];
+
+    __weak RCTModalHostView *weakSelf = self;
+    _modalViewController.boundsDidChangeBlock = ^(CGRect newBounds) {
+      [weakSelf notifyForBoundsChange:newBounds];
+    };
+  }
+
+  return self;
+}
+
+- (void)notifyForBoundsChange:(CGRect)newBounds
+{
+  if (_hasModalView) {
+    [_bridge.uiManager setFrame:newBounds forView:_modalViewController.view];
+  }
+}
+
+- (NSArray *)reactSubviews
+{
+  return _hasModalView ? @[_modalViewController.view] : @[];
+}
+
+- (void)insertReactSubview:(UIView *)subview atIndex:(__unused NSInteger)atIndex
+{
+  [subview addGestureRecognizer:_touchHandler];
+  _modalViewController.view = subview;
+  _hasModalView = YES;
+}
+
+- (void)removeReactSubview:(UIView *)subview
+{
+  RCTAssert(subview == _modalViewController.view, @"Cannot remove view other than modal view");
+  _modalViewController.view = nil;
+  _hasModalView = NO;
+}
+
+- (void)didMoveToSuperview
+{
+  [super didMoveToSuperview];
+
+  if (self.superview) {
+    [self.backingViewController presentViewController:_modalViewController animated:self.animated completion:nil];
+  } else {
+    [_modalViewController dismissViewControllerAnimated:self.animated completion:nil];
+  }
+}
+
+@end