runtime[NativeObject]: protect icls against changes
runtime[NativeObject]: protect icls against changes

We only allow setting it to something different from its default
value (ICLS_OBJECT) and prohibit further changes.

tmikov/jscomp,tmikov/jscomp,tmikov/jscomp,tmikov/jscomp,tmikov/jscomp,tmikov/jscomp,tmikov/jscomp,tmikov/jscomp,tmikov/jscomp