message: Fix HTMLImports typing
diff: @@ -16,7 +16,7 @@ interface Window {
  define: ((deps: string[], moduleBody: OnExecutedCallback) => void)&{
  _reset?: () => void;
  };
  - HTMLImports: { importForElement: (element: Element) => HTMLLinkElement | undefined }
  + HTMLImports?: { importForElement: (element: Element) => HTMLLinkElement | undefined };
  }
  type OnExecutedCallback = (...args: Array<{}>) => void;
  