Minor: moved test for check the iterating over String to common place.

 #KT-3617 Fixed
JS frontend: suppress UNUSED_PARAM for native declarations.

 #KT-2141 Fixed
JS frontend: suppress warnings from stdlib.
Extract the suppressing logic for debugger.
JS tests: create JetFiles by JetPsiFactory instead of by PsiManager to avoid to access to PsiManager after it disposed.

That fixed js tests after changes in DiagnosticsWithSuppression.
Reset extensions cache in DiagnosticsWithSuppression when Application changed(that hepens in tests).
Introduce constant for "kt".
Use JarFileSystem for collect kotlin files inside archive instead of own implementation.
K2JS: introduce "no-stdlb" parameter and use stdlib from compiler by default.
Workaround for bootstrapping with changes in kotlin2js ant task and cli compiler.
K2JS: test that UNUSED_PARAM warnings inside native declarations and all warnings from stdlib was suppressed.