JS: refactor flat declaration generator. Add more tests on inter-module dependencies

js/js.translator/testData/box/crossModuleRef/objectIsObject.kt

@@ -0,0 +1,19 @@
+// MODULE: lib
+// FILE: lib.kt
+package lib
+
+object O
+
+// MODULE: main(lib)
+// FILE: main.kt
+package main
+
+import lib.*
+
+fun box(): String {
+    var o: Any = O
+    if (o !is O) return "fail1"
+    if (!(o is O)) return "fail2"
+
+    return "OK"
+}