message: Implement IndexedDBSource#upgrade to align with Source#upgrade
diff: @@ -62,8 +62,10 @@ export default class IndexedDBSource extends Source implements Pullable, Pushabl
  super(settings);
  this._namespace = settings.namespace || 'orbit';
  + }
  - this.schema.on('upgrade', () => this.reopenDB());
  + upgrade(): Promise<void> {
  + return this.reopenDB();
  }
  /**
  