Scanner: remove PushBack calls when we're going to return ILLEGAL.

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index a9da8514023968bbce698d3ee2b26645a4e5bdd7..b15ccc6e42a04b469d69ba4aa89a51ec4f27fd83 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -20,6 +20,7 @@ var kMessages = {
   unexpected_strict_reserved:    ["Unexpected strict mode reserved word"],
   unexpected_eos:                ["Unexpected end of input"],
   malformed_regexp:              ["Invalid regular expression: /", "%0", "/: ", "%1"],
+  malformed_regexp_flags:        ["Invalid regular expression flags"],
   unterminated_regexp:           ["Invalid regular expression: missing /"],
   regexp_flags:                  ["Cannot supply flags when constructing one RegExp from another"],
   incompatible_method_receiver:  ["Method ", "%0", " called on incompatible receiver ", "%1"],