[JS IR] Support per-file mode and ES modules

js/js.translator/testData/box/esModules/jsModule/externalClass.kt

@@ -0,0 +1,33 @@
+// EXPECTED_REACHABLE_NODES: 1284
+// ES_MODULES
+// DONT_TARGET_EXACT_BACKEND: JS
+
+package foo
+
+@JsModule("./externalClass.mjs")
+external class A(x: Int = definedExternally) {
+    val x: Int
+
+    fun foo(y: Int): Int = definedExternally
+
+    fun bar(vararg arg: String): String = definedExternally
+}
+
+class C {
+    val e = arrayOf("e")
+    val f = arrayOf("f")
+    val a = A(1)
+
+    fun qux() = a.bar(*e, *f)
+}
+
+
+fun box(): String {
+    val a = A(23)
+    assertEquals(23, a.x)
+    assertEquals(65, a.foo(42))
+
+    assertEquals(C().qux(), "(ef)")
+
+    return "OK"
+}