[es6] Tail calls support.

src/parsing/parser.cc

Index: src/parsing/parser.cc
diff --git a/src/parsing/parser.cc b/src/parsing/parser.cc
index dd9744e7f6314d72f1b3b91ad4b28218da7d1225..c3ff9079b21ad232713d4e2c631f770504ac072d 100644
--- a/src/parsing/parser.cc
+++ b/src/parsing/parser.cc
@@ -2807,7 +2807,10 @@ Statement* Parser::ParseReturnStatement(bool* ok) {
           is_object_conditional, pos);
     }
 
-    return_value->MarkTail();
+    // ES6 14.6.1 Static Semantics: IsInTailPosition
+    if (FLAG_harmony_tailcalls && !is_sloppy(language_mode())) {
+      return_value->MarkTail();
+    }
   }
   ExpectSemicolon(CHECK_OK);