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>