Implement ES2015 class feature (part II.) (#2439)

This patch is the second milestone of the implementation of this new language element.

Supported:
 - Single class inheritance
 - Functionality of 'super' keyword
 - Implicit constructor in class heritage
 - Specific behaviour while extending with the built-in 'Array' or '%TypedArray%' object
 - Abstract subclasses (Mix-ins)

JerryScript-DCO-1.0-Signed-off-by: Robert Fancsik frobert@inf.u-szeged.hu

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

@@ -911,6 +911,10 @@ parser_error_to_string (parser_error_t error) /**< error code */
     {
       return "Classes may not have a static property called 'prototype'.";
     }
+    case PARSER_ERR_UNEXPECTED_SUPER_REFERENCE:
+    {
+      return "Super is not allowed to be used here.";
+    }
 #endif /* !CONFIG_DISABLE_ES2015_CLASS */
     case PARSER_ERR_LEFT_PAREN_EXPECTED:
     {