RN: Create Experiment for `collapsable` (iOS)

Summary:
Sets up an experiment that enables `collapsable` in Fabric for iOS. This will enable us to validate with production data that enabling the use of this prop does not cause unexpected regressions.

Changelog:
[internal]

Reviewed By: mdvacca

Differential Revision: D27987619

fbshipit-source-id: 9b1c0ff45bed09b1e72ad7d9c782f07bb4211cc6

Libraries/Components/View/ViewInjection.js

@@ -0,0 +1,13 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict
+ * @format
+ */
+
+export default {
+  unstable_enableCollapsable: false,
+};

Libraries/Components/View/ViewNativeComponent.js

@@ -13,8 +13,10 @@ import {type HostComponent} from '../../Renderer/shims/ReactNativeTypes';
 import Platform from '../../Utilities/Platform';
 import codegenNativeCommands from '../../Utilities/codegenNativeCommands';
 import ReactNativeViewViewConfigAndroid from './ReactNativeViewViewConfigAndroid';
+import ViewInjection from './ViewInjection';
 import {type ViewProps as Props} from './ViewPropTypes';
 import * as React from 'react';
+import ReactNativeViewConfigRegistry from '../../Renderer/shims/ReactNativeViewConfigRegistry';
 
 const ViewNativeComponent: HostComponent<Props> = NativeComponentRegistry.get<Props>(
   'RCTView',
@@ -24,6 +26,14 @@ const ViewNativeComponent: HostComponent<Props> = NativeComponentRegistry.get<Pr
       : {uiViewClassName: 'RCTView'},
 );
 
+if (Platform.OS === 'ios') {
+  if (ViewInjection.unstable_enableCollapsable) {
+    const viewConfig = ReactNativeViewConfigRegistry.get('RCTView');
+    // $FlowFixMe - Yes, knowingly writing to a read-only property.
+    viewConfig.validAttributes.collapsable = true;
+  }
+}
+
 interface NativeCommands {
   +hotspotUpdate: (
     viewRef: React.ElementRef<HostComponent<mixed>>,