function isGlobalCliUsingYarn(projectDir) {
  return fs.existsSync(path.join(projectDir, 'yarn.lock'));
}
Check that 'react-native init' itself used yarn to install React Native.
When using an old global react-native-cli@1.0.0 (or older), we don't want
to install React Native with npm, and React + Jest with yarn.
Let's be safe and not mix yarn and npm in a single project.
@param projectDir e.g. /Users/martin/AwesomeApp