RN: Create `RootTag` Type

Summary:
Creates a `RootTag` type and refactors the `RootTagContext` module a bit.

This creates space for eventually changing `RootTag` into an opaque type that is only created once by `AppContainer`, and only consumed by native abstractions.

Changelog:
[Internal]

(Note: this ignores all push blocking failures!)

Reviewed By: cpojer

Differential Revision: D21127173

fbshipit-source-id: 60177a6e5e02d6308e87f76d12a271114f8f8fe0

Libraries/ReactNative/AppContainer.js

@@ -14,7 +14,7 @@ const EmitterSubscription = require('../vendor/emitter/EmitterSubscription');
 const PropTypes = require('prop-types');
 const RCTDeviceEventEmitter = require('../EventEmitter/RCTDeviceEventEmitter');
 const React = require('react');
-const RootTagContext = require('./RootTagContext');
+import {RootTagContext, createRootTag} from './RootTag';
 const StyleSheet = require('../StyleSheet/StyleSheet');
 const View = require('../Components/View/View');
 
@@ -128,7 +128,7 @@ class AppContainer extends React.Component<Props, State> {
       );
     }
     return (
-      <RootTagContext.Provider value={this.props.rootTag}>
+      <RootTagContext.Provider value={createRootTag(this.props.rootTag)}>
         <View style={styles.appContainer} pointerEvents="box-none">
           {!this.state.hasError && innerView}
           {this.state.inspector}

Libraries/ReactNative/RootTag.js

@@ -0,0 +1,27 @@
+/**
+ * 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
+ */
+
+'use strict';
+
+import * as React from 'react';
+
+// TODO: Make this into an opaque type.
+export type RootTag = number;
+
+export const RootTagContext: React$Context<RootTag> = React.createContext<RootTag>(
+  0,
+);
+
+/**
+ * Intended to only be used by `AppContainer`.
+ */
+export function createRootTag(rootTag: number): RootTag {
+  return rootTag;
+}

Libraries/ReactNative/RootTagContext.js

@@ -1,15 +0,0 @@
-/**
- * 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
- */
-
-'use strict';
-
-const React = require('react');
-
-module.exports = (React.createContext<number>(0): React$Context<number>);

index.js

@@ -95,7 +95,7 @@ import typeof Platform from './Libraries/Utilities/Platform';
 import typeof processColor from './Libraries/StyleSheet/processColor';
 import typeof {PlatformColor} from './Libraries/StyleSheet/PlatformColorValueTypes';
 import typeof {DynamicColorIOS} from './Libraries/StyleSheet/PlatformColorValueTypesIOS';
-import typeof RootTagContext from './Libraries/ReactNative/RootTagContext';
+import typeof {RootTagContext} from './Libraries/ReactNative/RootTag';
 import typeof DeprecatedColorPropType from './Libraries/DeprecatedPropTypes/DeprecatedColorPropType';
 import typeof DeprecatedEdgeInsetsPropType from './Libraries/DeprecatedPropTypes/DeprecatedEdgeInsetsPropType';
 import typeof DeprecatedPointPropType from './Libraries/DeprecatedPropTypes/DeprecatedPointPropType';
@@ -468,7 +468,7 @@ module.exports = {
     return require('./Libraries/ReactNative/requireNativeComponent');
   },
   get unstable_RootTagContext(): RootTagContext {
-    return require('./Libraries/ReactNative/RootTagContext');
+    return require('./Libraries/ReactNative/RootTag').RootTagContext;
   },
   get unstable_enableLogBox(): () => void {
     return () =>