Drag and Drop list ordering issue

#### What is the expected behavior?
I am trying to reproduce the simple drag and drop list ordering example from here: https://material.angular.io/cdk/drag-drop/overview

#### What is the current behavior?
However, the dragged item always snaps back to its original place. The ordering animation is showing. The event shows that the previous and the current index are always 0.
https://i.imgur.com/1RkV6MT.gifv
https://i.imgur.com/k4wMbt3.gifv

#### What are the steps to reproduce?
The drop listener and the test data:
```
strings = ["dsfs","fgjdf","ghama","jéép"];
drop(event: CdkDragDrop) {
console.log(event);
moveItemInArray(this.strings, event.previousIndex, event.currentIndex);
}
```
The HTML:
```

```
The CSS is copy-paste from the tutorial
#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular: 6.2.1
Material: 7.2.1
OS: Ubuntu 16.04
TypeScript: 3.2.2
Chrome: 65.0.3325.146

Comienza con la reproducción del issue y compara su markup con la descripción general de arrastrar y soltar de Angular Material CDK. Inspecciona la configuración de cdkDropList y cdkDrag y, a continuación, verifica previousIndex y currentIndex del evento de soltar mientras arrastras entre elementos. Se considera terminado cuando el elemento arrastrado permanece en su nueva posición y los datos de la lista se reordenan.

Escrito por el modelo de indexación a partir del texto del issue.