Asset Key property type
The Asset Key property type is just like a simple String property type, but it allows to open a dialog with all the Asset Pack file keys defined in the project. Then, when you select a key in the dialog, it will be set as value to the property.

The scene compiler generates Asset Key properties like this:
class Dragon extends Phaser.GameObjects.Sprite {
constructor(..) {
...
}
/** @type {string} */
myAssetKey = "acorn-3";
}