Support let identifier in non-strict mode. (#3427)

This code adds a lot of checks and complexity to the code.

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

jerry-core/parser/js/js-parser.h

@@ -120,7 +120,8 @@ typedef enum
   PARSER_ERR_OBJECT_PROPERTY_REDEFINED,               /**< property of object literal redefined */
 #if ENABLED (JERRY_ES2015)
   PARSER_ERR_VARIABLE_REDECLARED,                     /**< a variable redeclared */
-  PARSER_ERR_LEXICAL_SINGLE_STATEMENT,                /**< lexical variable in single statement context */
+  PARSER_ERR_LEXICAL_SINGLE_STATEMENT,                /**< lexical declaration in single statement context */
+  PARSER_ERR_LEXICAL_LET_BINDING,                     /**< let binding cannot be declared in let/const */
   PARSER_ERR_MISSING_ASSIGN_AFTER_CONST,              /**< an assignment is required after a const declaration */
 
   PARSER_ERR_MULTIPLE_CLASS_CONSTRUCTORS,             /**< multiple class constructor */