The type of the property value.
This is determined from the type argument
T used to instantiate the
DiagnosticsProperty class. This means that the type is available even if
value is null, but it also means that the propertyType is only as
accurate as the type provided when invoking the constructor.
Generally, this is only useful for diagnostic tools that should display null values in a manner consistent with the property type. For example, a tool might display a null Color value as an empty rectangle instead of the word "null".
Type get propertyType => T;