Method preloadExtension

src/service/extensions-impl.js:214–221

* Returns the promise that will be resolved when the extension has been * loaded. If necessary, adds the extension script to the page. * @param {string} extensionId * @param {string=} version * @return {!Promise<!ExtensionDef>}

(extensionId, version = DEFAULT_VERSION)

   * @return {!Promise<!ExtensionDef>}
   */
  preloadExtension(extensionId, version = DEFAULT_VERSION) {
    if (extensionId == 'amp-embed') {
      extensionId = 'amp-ad';
    }
    const holder = this.getExtensionHolder_(extensionId, version);
    this.insertExtensionScriptIfNeeded_(extensionId, version, holder);
    return this.waitFor_(holder);
  }

  /**
   * Returns the promise that will be resolved when the extension has been