I am working with a huge 3rdparty library (Babylon JS) that will be served from its own CDN and cannot be included in my Closure Compiler run.
The library contains one object and everything defined as parts of it.
It has no externs file available so I started to write one but it is growing quickly. It would be easier to just tell Closure Compiler to not mangle any properties I am setting, including the ones I am setting on objects created by constructors on the object.
EDIT:
Added the name of the library.
The Closure Compiler has no feature that would allow you to say "don't rename any property on this object" except to disable property renaming entirely. The general idea is that it would be very easy for an "unrenamable object" to leak into a loosely typed value ('unknown', Object, etc) and disable renaming for the entire program. And that would make maintaining the expected optimizations for larger projects difficult. However, that is certainly something I would like the team to revisit at some point.