`NSDate.timeIntervalSinceReferenceDate` is declared as both a type property and an instance property

[liamappelbe]

While cleaning up the "duplicate method" error spam, I found that there are two properties on NSDate called timeIntervalSinceReferenceDate. One is an instance property and the other is a type property.

This is legal in ObjC, but in Dart having an instance method with the same name as a static method is illegal. So we'll need to dedupe these methods by changing one of their names.