Method handler

src/cdk/drag-drop/drag-ref.ts:1199–1210

(event: TransitionEvent)

    return this._ngZone.runOutsideAngular(() => {
      return new Promise(resolve => {
        const handler = (event: TransitionEvent) => {
          if (
            !event ||
            (this._preview &&
              _getEventTarget(event) === this._preview.element &&
              event.propertyName === 'transform')
          ) {
            cleanupListener();
            resolve();
            clearTimeout(timeout);
          }
        };

        // If a transition is short enough, the browser might not fire the `transitionend` event.
        // Since we know how long it's supposed to take, add a timeout with a 50% buffer that'll