Fabric: Element<X>::children() now accepts children of any Element<> type

Summary:
Before this change `Element<X>` cannot have children of `Element<Y>` which was wrong because we don't have such limitation in Fabric.

Changelog: [Internal] Fabric-specific internal change.

Reviewed By: sammy-SC

Differential Revision: D19596283

fbshipit-source-id: 9002f5dd42b3d05e7cf492499499399c97b58152

ReactCommon/fabric/element/Element.h

@@ -48,6 +48,13 @@ class Element final {
     fragment_.props = ShadowNodeT::defaultSharedProps();
   }
 
+  /*
+   * Converts to `ElementFragment` object.
+   */
+  operator ElementFragment() {
+    return fragment_;
+  }
+
   /*
    * Sets `tag`.
    */
@@ -83,11 +90,11 @@ class Element final {
   /*
    * Sets children.
    */
-  Element &children(std::vector<Element> children) {
+  Element &children(std::vector<ElementFragment> children) {
     auto fragments = ElementFragment::List{};
     fragments.reserve(children.size());
     for (auto const &child : children) {
-      fragments.push_back(child.fragment_);
+      fragments.push_back(child);
     }
     fragment_.children = fragments;
     return *this;

ReactCommon/fabric/element/tests/ElementTest.cpp

@@ -9,6 +9,7 @@
 
 #include <gtest/gtest.h>
 
+#include <react/components/root/RootComponentDescriptor.h>
 #include <react/components/view/ViewComponentDescriptor.h>
 #include <react/element/ComponentBuilder.h>
 #include <react/element/Element.h>
@@ -20,7 +21,7 @@ using namespace facebook::react;
 TEST(ElementTest, testNormalCases) {
   auto builder = simpleComponentBuilder();
 
-  auto shadowNodeA = std::shared_ptr<ViewShadowNode const>{};
+  auto shadowNodeA = std::shared_ptr<RootShadowNode const>{};
   auto shadowNodeAA = std::shared_ptr<ViewShadowNode const>{};
   auto shadowNodeAB = std::shared_ptr<ViewShadowNode const>{};
   auto shadowNodeABA = std::shared_ptr<ViewShadowNode const>{};
@@ -30,16 +31,15 @@ TEST(ElementTest, testNormalCases) {
 
   // clang-format off
   auto element =
-      Element<ViewShadowNode>()
+      Element<RootShadowNode>()
         .reference(shadowNodeA)
         .tag(1)
         .props([]() {
-          auto props = std::make_shared<ViewProps>();
-          props->zIndex = 42;
+          auto props = std::make_shared<RootProps>();
           props->nativeId = "node A";
           return props;
         })
-        .finalize([](ViewShadowNode &shadowNode){
+        .finalize([](RootShadowNode &shadowNode){
           shadowNode.sealRecursive();
         })
         .children({