Implement experimental exponentiation operator

src/scanner.h

Index: src/scanner.h
diff --git a/src/scanner.h b/src/scanner.h
index 86a0098f86c0b4c97b4958148c4f6d27f04ce9b8..8c30871a6d1354369cc9d2746f826603cbeb94c2 100644
--- a/src/scanner.h
+++ b/src/scanner.h
@@ -470,6 +470,8 @@ class Scanner {
   void SetHarmonyTemplates(bool templates) { harmony_templates_ = templates; }
   bool HarmonyUnicode() const { return harmony_unicode_; }
   void SetHarmonyUnicode(bool unicode) { harmony_unicode_ = unicode; }
+  bool HarmonyExponentiation() const { return harmony_exponentiation_; }
+  void SetHarmonyExponentiation(bool exp) { harmony_exponentiation_ = exp; }
 
   // Returns true if there was a line terminator before the peek'ed token,
   // possibly inside a multi-line comment.
@@ -744,6 +746,8 @@ class Scanner {
   bool harmony_templates_;
   // Whether we allow \u{xxxxx}.
   bool harmony_unicode_;
+  // Whether we allow x ** y or x **= y
+  bool harmony_exponentiation_;
 };
 
 } }  // namespace v8::internal