DownCastBuilder(description: String, subType: KClass<TSub>, commonFields: CommonFields<T?>)
description - The description of this down-cast; will be used for the creation of the IAssertion.
subType - The resulting type of the down-cast.
commonFields - The IAssertionPlantWithCommonFields.CommonFields of the
IAssertionPlant/IAssertionPlantNullable which uses this DownCastBuilder.
The down-cast will be performed on its subject.
Moreover, the containing information will inter alia be used in reporting.
Constructor