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

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

@@ -0,0 +1,45 @@
+// DONT_TARGET_EXACT_BACKEND: JS
+// ES_MODULES
+// FILE: a.kt
+@file:JsModule("./externalPackage.mjs")
+package foo
+
+external class A(x: Int) {
+    val x: Int
+
+    fun foo(y: Int): Int = definedExternally
+
+    class Nested {
+        val y: Int
+    }
+}
+
+external object B {
+    val x: Int = definedExternally
+
+    fun foo(y: Int): Int = definedExternally
+}
+
+external fun foo(y: Int): Int = definedExternally
+
+external val bar: Int = definedExternally
+
+// FILE: b.kt
+package foo
+
+fun box(): String {
+    val a = A(23)
+    assertEquals(23, a.x)
+    assertEquals(65, a.foo(42))
+
+    val nested = A.Nested()
+    assertEquals(55, nested.y)
+
+    assertEquals(123, B.x)
+    assertEquals(265, B.foo(142))
+
+    assertEquals(365, foo(42))
+    assertEquals(423, bar)
+
+    return "OK"
+}