Extend scanner with new Harmony module keywords (under flag).

src/parser.h

Index: src/parser.h
diff --git a/src/parser.h b/src/parser.h
index bc2af61d41a44f141a1a1737b1b21bd38b2d5bd5..fbc4a1529d71efe2372d5ce1fd34f23b0c536f7e 100644
--- a/src/parser.h
+++ b/src/parser.h
@@ -807,6 +807,7 @@ class Parser {
   Mode mode_;
   bool allow_natives_syntax_;
   bool allow_lazy_;
+  bool allow_modules_;
   bool stack_overflow_;
   // If true, the next (and immediately following) function literal is
   // preceded by a parenthesis.