message: Use async/await to simplify Source#transformed
  Also fixes typing issues.
diff: @@ -197,19 +197,15 @@ export abstract class Source implements Evented, Performer {
  */
  async transformed(transforms: Transform[]): Promise<Transform[]> {
  await this.activated;
  - return transforms
  - .reduce((chain, transform) => {
  - return chain.then(() => {
  - if (this._transformLog.contains(transform.id)) {
  - return Promise.resolve();
  +
  + for (let transform of transforms) {
  + if (!this._transformLog.contains(transform.id)) {
  + await this._transformLog.append(transform.id);
  + await settleInSeries(this, 'transform', transform);
  + }
  }
  - return this._transformLog
  - .append(transform.id)
  - .then(() => settleInSeries(this, 'transform', transform));
  - });
  - }, Promise.resolve())
  - .then(() => transforms);
  + return transforms;
  }
  /////////////////////////////////////////////////////////////////////////////
  