Optimize snapshot operations to return special collection implementations when result is empty or has single element.

#KT-9990 Fixed

js/js.libraries/src/core/kotlin.kt

@@ -91,6 +91,8 @@ internal fun <T> arrayPlusCollection(array: dynamic, collection: Collection<T>):
     return result
 }
 
+// no singleton map implementation in js, return map as is
+internal inline fun <K, V> Map<K, V>.toSingletonMap(): Map<K, V> = this
 
 internal inline fun <T> Array<out T>.copyToArrayOfAny(isVarargs: Boolean): Array<out Any?> =
         if (isVarargs)