[JS BE] Extract backend specific code from shared stdlib sources

js/js.translator/testData/box/dynamic/iterator.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JS_IR
 // KJS_WITH_FULL_RUNTIME
 // EXPECTED_REACHABLE_NODES: 1556
 package foo

libraries/stdlib/js-v1/src/kotlin/currentBeMisc.kt

@@ -9,4 +9,6 @@ package kotlin.js
 // Used for js.translator/testData/box/number/mulInt32.kt
 @library
 @JsName("imulEmulated")
-internal fun imul(x: Int, y: Int): Int = definedExternally
+internal fun imul(x: Int, y: Int): Int = definedExternally
+
+internal inline fun isArrayish(o: dynamic) = js("Kotlin").isArrayish(o)

libraries/stdlib/js/src/kotlin/collections/ArraysJs.kt

@@ -14,7 +14,7 @@ internal fun <T> Array<out T>.contentDeepHashCodeImpl(): Int {
     for (element in this) {
         val elementHash = when {
             element == null -> 0
-            js("Kotlin").isArrayish(element) -> (element.unsafeCast<Array<*>>()).contentDeepHashCodeImpl()
+            isArrayish(element) -> (element.unsafeCast<Array<*>>()).contentDeepHashCodeImpl()
 
             element is UByteArray   -> element.contentHashCode()
             element is UShortArray  -> element.contentHashCode()

libraries/stdlib/js/src/kotlin/dynamic.kt

@@ -38,7 +38,7 @@ public operator fun dynamic.iterator(): Iterator<dynamic> {
     return when {
         this["iterator"] != null ->
             this["iterator"]()
-        js("Kotlin").isArrayish(r) ->
+        isArrayish(r) ->
             r.unsafeCast<Array<*>>().iterator()
 
         else ->