An asset is a resource that is loaded asynchronously.
loaded<D>(data: D ): Asset<D>
data: D | null
error: Error | null
onLoad(action: (data: D )=>void ): this
onError(action: (err: Error )=>void ): this
onFinish(action: ()=>void ): this
then(action: (data: D )=>void ): Asset<D>
catch(action: (err: Error )=>void ): Asset<D>
finally(action: ()=>void ): Asset<D>