[es6] Correct parsing of regular expression literal flags.

src/scanner.h

Index: src/scanner.h
diff --git a/src/scanner.h b/src/scanner.h
index ab5693ee5a5c3fc889ec2cec80561fa61c29ae9e..8a39bd918bf98b2450d8f49640699896c388c80f 100644
--- a/src/scanner.h
+++ b/src/scanner.h
@@ -445,9 +445,8 @@ class Scanner {
   // Scans the input as a regular expression pattern, previous
   // character(s) must be /(=). Returns true if a pattern is scanned.
   bool ScanRegExpPattern(bool seen_equal);
-  // Returns true if regexp flags are scanned (always since flags can
-  // be empty).
-  bool ScanRegExpFlags();
+  // Scans the input as regular expression flags. Returns the flags on success.
+  Maybe<RegExp::Flags> ScanRegExpFlags();
 
   // Scans the input as a template literal
   Token::Value ScanTemplateStart();