aot compiler

This error appears because of the AOT-compiler.

There are two compilers in Angular:
JIT (Just-in-Time): It's for local machines and faster than AOT compiler, but special features like tree shaking are not enabled.
AOT(Ahead-of-Time): It's compiler used for prod builds. This compiler enables for example tree shaking, which takes more time during compiling but decreases loading time of the application by a lot.

And this AOT compiler doesn't allow private symbols because he can only interact with exported members. Decorated and data bound properties must be public.