function preparePlugin(pluginDir, isLocal) {
  console.log(`Preparing ${pluginDir} plugin for deployment.`);
  createPluginPage(pluginDir, isLocal);
  createReadmePage(pluginDir, isLocal);
  return gulp
    .src(['./plugins/' + pluginDir + '/build/test_bundle.js'], {
      base: './plugins/',
      allowEmpty: true,
    })
    .pipe(gulp.dest('./gh-pages/plugins/'));
}
Copy over files needed to deploy this plugin and its test page to
github pages.
@param {string} pluginDir The directory with the plugin source files.
@param {boolean} isLocal True if building for a local test. False if
    building for gh-pages.
@returns {*} gulp stream