Fix some problems with jsCode function

1. Fix a bug in frontend when passing non-String constant led to exception
2. Fix a bug in backend when passing non-JetStringTeplate string constant led to exception
3. Avoid recomputing constant argument in backend

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

@@ -0,0 +1,14 @@
+package foo
+
+fun box(): String {
+    var c: Int = 0
+
+    val code = "c = 3"
+    js(code)
+
+    assertEquals(3, c)
+    js(("c = 5"))
+
+    assertEquals(5, c)
+    return "OK"
+}