RN 服务器起来但无法gradlew

Could not install the app on the device, read the error above for details.
Make sure you have an Android emulator running or a device connected and have
set up your Android development environment:
https://facebook.github.io/react-native/docs/android-setup.html

是因为clone了项目，而android下的gradlew无执行（X）权限。

解决：
cd android
chmod +x gradlew