Support PRIVATE_TO_THIS in cls stub builder
Reintroduce change erroneously reverted in 2fec18ffa7e620f386a76f7eaa2cd2589614c8f0
JS backend: tests for KT-6401 JS: wrong result for code with many spread operator
JS backend: fix KT-6401 JS: wrong result for code with many spread operator

#KT-6401 Fixed