JS backend: added jsCode test cases

js/js.translator/testData/jsCode/cases/object.kt

@@ -0,0 +1,20 @@
+package foo
+
+native trait Summizer {
+    fun sum(a: Int, b: Int): Int
+}
+
+fun getSummizer(): Summizer = js("""
+    var summizer = {
+        sum: function(a, b) { return a + b;}
+    };
+
+    return summizer;
+""");
+
+fun box(): String {
+    val summizer = getSummizer()
+    assertEquals(3, summizer.sum(1, 2))
+
+    return "OK"
+}