[JS] Keep interfaces

^KT-54173 fixed

js/js.translator/testData/box/keep/keepInterface.kt

@@ -0,0 +1,34 @@
+// TARGET_BACKEND: JS_IR
+// ONLY_IR_DCE
+// RUN_PLAIN_BOX_FUNCTION
+// INFER_MAIN_MODULE
+// KEEP: DenotedFace
+
+// MODULE: keep_interface
+// FILE: lib.kt
+
+interface DenotedFace {
+    fun keepMeA() = 7
+    fun keepMeB(): Int
+}
+
+class A : DenotedFace {
+    override fun keepMeB(): Int {
+        return 8
+    }
+}
+
+@JsExport
+fun foo(): A {
+    return A()
+}
+
+// FILE: test.js
+function box() {
+    var a = this["keep_interface"].foo()
+
+    if (a.keepMeA_ds0nq4_k$() != 7) return "fail 1"
+    if (a.keepMeB_ds0nq3_k$() != 8) return "fail 2"
+
+    return "OK"
+}