Add support for function argument initializers. (#2571)

EcmaScript 2015 14.1.

Note: arrow functions with default arguments are not supported yet.

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

jerry-core/parser/js/js-parser-util.c

@@ -1038,6 +1038,12 @@ parser_error_to_string (parser_error_t error) /**< error code */
     {
       return "Duplicated label.";
     }
+#ifndef CONFIG_DISABLE_ES2015_FUNCTION_PARAMETER_INITIALIZER
+    case PARSER_ERR_DUPLICATED_ARGUMENT_NAMES:
+    {
+      return "Duplicated function argument names are not allowed here.";
+    }
+#endif /* !CONFIG_DISABLE_ES2015_FUNCTION_PARAMETER_INITIALIZER */
     case PARSER_ERR_OBJECT_PROPERTY_REDEFINED:
     {
       return "Property of object literal redefined.";