function getBrowser(id) {
    if (!id) {
      // No ID parameter?  No such browser.
      return null;
    }
    const browser = launcher._browsers.find((b) => b.id == id);
    if (!browser) {
      return null;
    }
    return browser;
  }
Find the browser associated with the "id" parameter of the request.
This ID was assigned by Karma when the browser was launched, and passed to
the web server from the Jasmine tests.

If the browser is not found, this function will return null.

@param {?string} id
@return {karma.Launcher.Browser|null}