webextensions-examples/embedded-webextension-sdk/step1-hybrid-addon/main.js

const webext = require("sdk/webextension");
const {setSyncLegacyDataPort} = require("./lib/user-data-storage");

webext.startup().then(({browser}) => {
  browser.runtime.onConnect.addListener(port => {
    if (port.name === "sync-legacy-addon-data") {
      setSyncLegacyDataPort(port);
    }
  });
});