>npm run eject

> @ eject C:\Users\Ryu\react\expo-backgroundTimer
> expo eject

We didn't find any uses of the Expo SDK in your project, so you should be fine to eject to "Plain" React Native.
(This check isn't very sophisticated, though.)

Warning! Your git working tree is dirty.
It's recommended to commit all your changes before proceeding,
so you can revert the changes made by this command if necessary.

? How would you like to eject your app?
  Read more: https://docs.expo.io/versions/latest/expokit/eject/ React Native
We have a couple of questions to ask you about how you'd like to name your app:
? What should your app appear as on a user's home screen? backgroundTimer
? What should your Android Studio and Xcode projects be called? backgroundTimer
Writing app.json...
Wrote to app.json, please update it manually in the future.
Generating the iOS folder.
Generating the Android folder.
Successfully copied template native code.
Updating your npm scripts in package.json...
Your package.json is up to date!

Note that using `npm start` will now require you to run Xcode and/or
Android Studio to build the native code for your project.
Removing node_modules...
Installing existing packages with npm...
npm WARN optional SKIPPING OPTIONAL DEPENDENCY: fsevents@1.2.7 (node_modules\fsevents):
npm WARN notsup SKIPPING OPTIONAL DEPENDENCY: Unsupported platform for fsevents@1.2.7: wanted {"os":"darwin","arch":"any"} (current: {"os":"win32","arch":"x64"})

Ejected successfully!

react-native linkを実行しようとするとエラーが発生する。

>react-native link
internal/modules/cjs/loader.js:605
    throw err;
    ^

Error: Cannot find module '@babel/runtime/helpers/interopRequireDefault'
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:603:15)
    at Function.Module._load (internal/modules/cjs/loader.js:529:25)
    at Module.require (internal/modules/cjs/loader.js:658:17)
    at require (internal/modules/cjs/helpers.js:22:18)
    at Object.<anonymous> (C:\Users\Ryu\react\expo-backgroundTimer\node_modules\react-native\local-cli\cliEntry.js:11:41)
    at Module._compile (internal/modules/cjs/loader.js:722:30)
    at Module._compile (C:\Users\Ryu\react\expo-backgroundTimer\node_modules\pirates\lib\index.js:99:24)
    at Module._extensions..js (internal/modules/cjs/loader.js:733:10)
    at Object.newLoader [as .js] (C:\Users\Ryu\react\expo-backgroundTimer\node_modules\pirates\lib\index.js:104:7)
    at Module.load (internal/modules/cjs/loader.js:620:32)

>npm g react-native-cli
undefined

C:\Users\Ryu\react\expo-backgroundTimer>npm install -g react-native-cli
C:\Program Files (x86)\Nodist\bin\react-native -> C:\Program Files (x86)\Nodist\bin\node_modules\react-native-cli\index.js
+ react-native-cli@2.0.1
updated 3 packages in 1.3s

react-native run-android
internal/modules/cjs/loader.js:605
    throw err;
    ^

Error: Cannot find module '@babel/runtime/helpers/interopRequireDefault'
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:603:15)
    at Function.Module._load (internal/modules/cjs/loader.js:529:25)
    at Module.require (internal/modules/cjs/loader.js:658:17)
    at require (internal/modules/cjs/helpers.js:22:18)
    at Object.<anonymous> (C:\Users\Ryu\react\expo-backgroundTimer\node_modules\react-native\local-cli\cliEntry.js:11:41)
    at Module._compile (internal/modules/cjs/loader.js:722:30)
    at Module._compile (C:\Users\Ryu\react\expo-backgroundTimer\node_modules\pirates\lib\index.js:99:24)
    at Module._extensions..js (internal/modules/cjs/loader.js:733:10)
    at Object.newLoader [as .js] (C:\Users\Ryu\react\expo-backgroundTimer\node_modules\pirates\lib\index.js:104:7)
    at Module.load (internal/modules/cjs/loader.js:620:32)

npm add @babel/runtime

を実行するとなおった。