Implement experimental exponentiation operator

src/preparser.h

Index: src/preparser.h
diff --git a/src/preparser.h b/src/preparser.h
index 8171f9433a4627c9b8fe5f1c5ac6a72d73a9ae4a..fa0fa90511338f38deb49067917f6ce6a45b9f41 100644
--- a/src/preparser.h
+++ b/src/preparser.h
@@ -117,7 +117,9 @@ class ParserBase : public Traits {
   bool allow_harmony_rest_params() const {
     return allow_harmony_rest_params_;
   }
-
+  bool allow_harmony_exponentiation() const {
+    return scanner()->HarmonyExponentiation();
+  }
   bool allow_strong_mode() const { return allow_strong_mode_; }
 
   // Setters that determine whether certain syntactical constructs are
@@ -157,6 +159,9 @@ class ParserBase : public Traits {
   void set_allow_harmony_rest_params(bool allow) {
     allow_harmony_rest_params_ = allow;
   }
+  void set_allow_harmony_exponentiation(bool allow) {
+    scanner()->SetHarmonyExponentiation(allow);
+  }
   void set_allow_strong_mode(bool allow) { allow_strong_mode_ = allow; }
 
  protected:
@@ -2460,7 +2465,15 @@ ParserBase<Traits>::ParseBinaryExpression(int prec, bool accept_IN, bool* ok) {
       Token::Value op = Next();
       Scanner::Location op_location = scanner()->location();
       int pos = position();
-      ExpressionT y = ParseBinaryExpression(prec1 + 1, accept_IN, CHECK_OK);
+      ExpressionT y = Traits::EmptyExpression();
+
+      if (op != Token::EXP) {
+        //  Left-to-right associativity
+        y = ParseBinaryExpression(prec1 + 1, accept_IN, CHECK_OK);
+      } else {
+        // Right-to-left associativity
+        y = ParseBinaryExpression(prec1, accept_IN, CHECK_OK);
+      }
 
       if (this->ShortcutNumericLiteralBinaryExpression(&x, y, op, pos,
                                                        factory())) {