add spec for FileReaderModule (#24904)

Summary:
Part of #24875.

## Changelog

[General] [Added] - add TM spec for FileReaderModule
Pull Request resolved: https://github.com/facebook/react-native/pull/24904

Reviewed By: fkgozali

Differential Revision: D15391738

Pulled By: rickhanlonii

fbshipit-source-id: 69e6ff53aba2d2227607905e1f70310bdd01d224

Libraries/Blob/FileReader.js

@@ -12,7 +12,7 @@
 
 const EventTarget = require('event-target-shim');
 const Blob = require('./Blob');
-const {FileReaderModule} = require('../BatchedBridge/NativeModules');
+import NativeFileReaderModule from './NativeFileReaderModule';
 
 type ReadyState =
   | 0 // EMPTY
@@ -87,7 +87,7 @@ class FileReader extends EventTarget(...READER_EVENTS) {
   readAsDataURL(blob: Blob) {
     this._aborted = false;
 
-    FileReaderModule.readAsDataURL(blob.data).then(
+    NativeFileReaderModule.readAsDataURL(blob.data).then(
       (text: string) => {
         if (this._aborted) {
           return;
@@ -108,7 +108,7 @@ class FileReader extends EventTarget(...READER_EVENTS) {
   readAsText(blob: Blob, encoding: string = 'UTF-8') {
     this._aborted = false;
 
-    FileReaderModule.readAsText(blob.data, encoding).then(
+    NativeFileReaderModule.readAsText(blob.data, encoding).then(
       (text: string) => {
         if (this._aborted) {
           return;

Libraries/Blob/NativeFileReaderModule.js

@@ -0,0 +1,21 @@
+/**
+ * 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
+ * @format
+ */
+
+'use strict';
+
+import type {TurboModule} from 'RCTExport';
+import * as TurboModuleRegistry from 'TurboModuleRegistry';
+
+export interface Spec extends TurboModule {
+  +readAsDataURL: (data: Object) => Promise<string>;
+  +readAsText: (data: Object, encoding: string) => Promise<string>;
+}
+
+export default TurboModuleRegistry.getEnforcing<Spec>('FileReaderModule');