JS backend: added jsCode test cases

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

@@ -0,0 +1,18 @@
+package foo
+
+fun countKeys(a: Array<Int>): Int = js("""
+    var result = 0;
+
+    for (var key in a) {
+        result += 1;
+    }
+
+    return result;
+""")
+
+fun box(): String {
+    assertEquals(3, countKeys(array(1,2,3)))
+    assertEquals(4, countKeys(array(1,2,3,4)))
+
+    return "OK"
+}