Add support for native pseudo-OS to Platform.select (#26966)

Summary:
When you write platform-specific code using [file extensions](https://facebook.github.io/react-native/docs/platform-specific-code#platform-specific-extensions), you can specify `.ios.js`, `.android.js`, or the catch-all `.native.js` when you are sharing code with a web project.

This `native` shortcut is missing for the `Platform.select` method, and this PR is adding support for that.

## Changelog

[General] [Added] - Platform.select now supports native as an option.
Pull Request resolved: #26966

Test Plan: Added relevant passing unit tests for Platform module.

Differential Revision: D18323670

Pulled By: cpojer

fbshipit-source-id: 7524c1914beab4f86041dcf8e60875380ebf7e02

Author: koke

Libraries/Utilities/Platform.android.js

@@ -12,8 +12,9 @@
 
 import NativePlatformConstantsAndroid from './NativePlatformConstantsAndroid';
 
-export type PlatformSelectSpec<A, D> = {
+export type PlatformSelectSpec<A, N, D> = {
   android?: A,
+  native?: N,
   default?: D,
 };
 
@@ -53,8 +54,12 @@ const Platform = {
   get isTV(): boolean {
     return this.constants.uiMode === 'tv';
   },
-  select: <A, D>(spec: PlatformSelectSpec<A, D>): A | D =>
-    'android' in spec ? spec.android : spec.default,
+  select: <A, N, D>(spec: PlatformSelectSpec<A, N, D>): A | N | D =>
+    'android' in spec
+      ? spec.android
+      : 'native' in spec
+      ? spec.native
+      : spec.default,
 };
 
 module.exports = Platform;

Libraries/Utilities/Platform.ios.js

@@ -12,8 +12,9 @@
 
 import NativePlatformConstantsIOS from './NativePlatformConstantsIOS';
 
-export type PlatformSelectSpec<D, I> = {
+export type PlatformSelectSpec<D, N, I> = {
   default?: D,
+  native?: N,
   ios?: I,
 };
 
@@ -59,8 +60,8 @@ const Platform = {
     }
     return false;
   },
-  select: <D, I>(spec: PlatformSelectSpec<D, I>): D | I =>
-    'ios' in spec ? spec.ios : spec.default,
+  select: <D, N, I>(spec: PlatformSelectSpec<D, N, I>): D | N | I =>
+    'ios' in spec ? spec.ios : 'native' in spec ? spec.native : spec.default,
 };
 
 module.exports = Platform;

Libraries/Utilities/__tests__/Platform-test.js

@@ -27,5 +27,17 @@ describe('Platform', () => {
       expect(PlatformIOS.select(obj)).toEqual(obj.ios);
       expect(PlatformAndroid.select(obj)).toEqual(obj.android);
     });
+
+    it('should return native value if no specific value was found', () => {
+      const obj = {native: 'native', default: 'default'};
+      expect(PlatformIOS.select(obj)).toEqual(obj.native);
+      expect(PlatformAndroid.select(obj)).toEqual(obj.native);
+    });
+
+    it('should return default value if no specific value was found', () => {
+      const obj = {default: 'default'};
+      expect(PlatformIOS.select(obj)).toEqual(obj.default);
+      expect(PlatformAndroid.select(obj)).toEqual(obj.default);
+    });
   });
 });