[es7] implement exponentiation operator proposal

src/parsing/preparser.h

Index: src/parsing/preparser.h
diff --git a/src/parsing/preparser.h b/src/parsing/preparser.h
index 3afaed115a8b8e93222e91412d3dcebd50720cc6..3b6afa7fd0d966ac8e687f53a225d621171d2402 100644
--- a/src/parsing/preparser.h
+++ b/src/parsing/preparser.h
@@ -902,6 +902,16 @@ class PreParserTraits {
 
   inline void RewriteDestructuringAssignments() {}
 
+  inline PreParserExpression RewriteExponentiation(PreParserExpression left,
+                                                   PreParserExpression right,
+                                                   int pos) {
+    return left;
+  }
+  inline PreParserExpression RewriteAssignExponentiation(
+      PreParserExpression left, PreParserExpression right, int pos) {
+    return left;
+  }
+
   inline void QueueDestructuringAssignmentForRewriting(PreParserExpression) {}
   inline void QueueNonPatternForRewriting(PreParserExpression) {}