- just legacy - report warning about deprecation
- both - report warning about deprecation of legacy
- no compiler explicitly chosen - error about explicit setting compiler
warn from cli legacy compiler
nowarn flag - kotlin.js.compiler.nowarn
KT-42326
KT-53074
Use "-Xno-check-impl" to suppress checking whether the platform declaration
implementation has the "impl" modifier.
Do not check presence of fake overrides from platform class in the impl class,
otherwise there would be a lot of errors about the fact that
equals/hashCode/toString are not marked with the "impl" modifier