This function will invoke the provided callback only when (and if) the legend Item in question is retrieved. If it's not retrieved because of the legend
module not being loaded or in case of some unforeseen error, it will simply do nothing. Programmers should be aware of this while calling this function.
Parameters
uniqueId: string
A uniqueId parameter which is of the form <mapServiceId.layerId> for layers or simply the mapService id in case of map services
callback: function
A callback function which will be invoked if, and when, the requested legend item becomes available.
This function will invoke the provided callback only when (and if) the legend Item in question is retrieved. If it's not retrieved because of the legend module not being loaded or in case of some unforeseen error, it will simply do nothing. Programmers should be aware of this while calling this function.