Support parsing async modifiers for functions. (#3460)

Only parsing is implemented, so the async functions currently behave
like normal function except they return with a resolved Promise object
when the function is terminated correctly.

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

jerry-core/vm/vm.c

@@ -2098,6 +2098,12 @@ vm_loop (vm_frame_ctx_t *frame_ctx_p) /**< frame context */
 
           goto error;
         }
+        case VM_OC_RETURN_PROMISE:
+        {
+          result = opfunc_return_promise (left_value);
+          left_value = ECMA_VALUE_UNDEFINED;
+          goto error;
+        }
         case VM_OC_STRING_CONCAT:
         {
           ecma_string_t *left_str_p = ecma_op_to_string (left_value);
@@ -2131,7 +2137,7 @@ vm_loop (vm_frame_ctx_t *frame_ctx_p) /**< frame context */
           *stack_top_p++ = ecma_copy_value (collection_p->buffer_p[tagged_idx]);
           continue;
         }
- #endif /* ENABLED (JERRY_ES2015) */
+#endif /* ENABLED (JERRY_ES2015) */
         case VM_OC_PUSH_ELISON:
         {
           *stack_top_p++ = ECMA_VALUE_ARRAY_HOLE;