[es6] Fix tail call elimination in single-expression arrow functions.

src/parsing/parser-base.h

Index: src/parsing/parser-base.h
diff --git a/src/parsing/parser-base.h b/src/parsing/parser-base.h
index 27b9764434c5364ff18c9cf561acde122b80090d..7f8fd719432a7b306308ece55599b4d0f441025d 100644
--- a/src/parsing/parser-base.h
+++ b/src/parsing/parser-base.h
@@ -2863,6 +2863,10 @@ ParserBase<Traits>::ParseArrowFunctionLiteral(
       body->Add(factory()->NewReturnStatement(expression, pos), zone());
       materialized_literal_count = function_state.materialized_literal_count();
       expected_property_count = function_state.expected_property_count();
+      // ES6 14.6.1 Static Semantics: IsInTailPosition
+      if (allow_tailcalls() && !is_sloppy(language_mode())) {
+        this->MarkTailPosition(expression);
+      }
     }
     super_loc = function_state.super_location();