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

issue の再現から始め、その markup を Angular Material CDK のドラッグ＆ドロップ概要と比較します。cdkDropList と cdkDrag の設定を調べ、項目間をドラッグしている間の drop event の previousIndex と currentIndex を確認します。完了とは、ドラッグした項目が新しい位置に残り、リストデータが並べ替えられている状態です。

索引モデルが issue の本文から書いたものです。