JS: when none of exhaustive when clauses match, generate code that throws exception. See KT-12194

js/js.libraries/src/core/builtins.kt

@@ -33,4 +33,7 @@ internal fun arrayIterator(array: dynamic): MutableIterator<dynamic> {
 }
 
 @JsName("PropertyMetadata")
-internal class PropertyMetadata(val name: String)
+internal class PropertyMetadata(val name: String)
+
+@JsName("noWhenBranchMatched")
+internal fun noWhenBranchMatched(): Nothing = throw NoWhenBranchMatchedException()

js/js.libraries/src/core/exceptions.kt

@@ -41,3 +41,5 @@ public open class ClassCastException(message: String? = null) : RuntimeException
 public open class AssertionError(message: String? = null) : Error(message)
 
 public open class NoSuchElementException(message: String? = null) : Exception(message)
+
+public open class NoWhenBranchMatchedException(message: String? = null) : RuntimeException(message)