Improve expected identifier checks. (#3064)

Checks for "of" or "from" does not accept quoted strings.

JerryScript-DCO-1.0-Signed-off-by: Zoltan Herczeg zherczeg.u-szeged@partner.samsung.com

tests/jerry/es2015/for-of.js

@@ -52,6 +52,10 @@ var forOf =
   "   obj[prop] += 4;"
 parse (forOf)
 
+var forOf =
+  "for (var a \"of\" []) {}"
+parse (forOf)
+
 checkError(5)
 
 var obj = {}