message: Make a few fields readonly
  As they're only set in the constructor.
diff: @@ -81,9 +81,9 @@ export interface OpenIDCredentials {
  * the given promise resolves.
  */
  export class WidgetApi extends EventEmitter {
  - private origin: string;
  + private readonly origin: string;
  private inFlightRequests: { [requestId: string]: (reply: FromWidgetRequest) => void } = {};
  - private readyPromise: Promise<any>;
  + private readonly readyPromise: Promise<any>;
  private readyPromiseResolve: () => void;
  private openIDCredentialsCallback: () => void;
  public openIDCredentials: OpenIDCredentials;
  