While it is nice to know that Unsafe is only used when no-arg ctors or instance 
creators are not present, it would be better to have a means to enforce it 
cannot be used.  Particularly in google app engine, I'd like to know that the 
code I use offline is safe inside GAE, rather than a delayed 
UnsupportedOperationException

Can you add GsonBuilder.disableUnsafe or similar?