[JS] Keep interfaces

^KT-54173 fixed

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

@@ -0,0 +1,26 @@
+// TARGET_BACKEND: JS_IR
+// ONLY_IR_DCE
+// RUN_PLAIN_BOX_FUNCTION
+// INFER_MAIN_MODULE
+// KEEP: DenotedFace
+
+// MODULE: keep_var
+// FILE: lib.kt
+
+interface DenotedFace {
+    fun keepMeA() = 7
+    fun keepMeB(): Int
+}
+
+@JsExport
+val denotedFaceTrigger = object : DenotedFace { override fun keepMeB() = 8 }
+
+// FILE: test.js
+function box() {
+    var a = this["keep_var"].denotedFaceTrigger
+
+    if (a.keepMeA_ds0nq4_k$() != 7) return "fail 1"
+    if (a.keepMeB_ds0nq3_k$() != 8) return "fail 2"
+
+    return "OK"
+}