message: add Task.empty
diff: @@ -68,6 +68,7 @@ namespace Task {
  }
  export function constant<T>(name: string, value: T): Task<T> { return create(name, async ctx => value); }
  + export function empty(): Task<void> { return create('', async ctx => {}); }
  export function fail(name: string, reason: string): Task<any> { return create(name, async ctx => { throw new Error(reason); }); }
  export interface Progress {
  