Code completion: do not list instance members from context with no instance

 #KT-6388 Fixed
 #KT-4422 Fixed
 #KT-5516 Fixed
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
~ fixes after review: add test for side effect + spread operator