[es6] Make assignment to new.target an early ReferenceError

src/preparser.cc

Index: src/preparser.cc
diff --git a/src/preparser.cc b/src/preparser.cc
index ec0ea2de3a04ba42bf5b32a54722420fb33de5d7..f186f8a694927d1808d48167b4de31340973c9d1 100644
--- a/src/preparser.cc
+++ b/src/preparser.cc
@@ -918,6 +918,8 @@ PreParser::Statement PreParser::ParseForStatement(bool* ok) {
           lhs.IsIdentifier() && lhs.AsIdentifier().IsLet();
       if (CheckInOrOf(lhs.IsIdentifier(), &mode, ok)) {
         if (!*ok) return Statement::Default();
+        // TODO(adamk): Should call CheckAndRewriteReferenceExpression here
+        // to catch early errors if lhs is not a valid reference expression.
         ParseExpression(true, CHECK_OK);
         Expect(Token::RPAREN, CHECK_OK);
         ParseSubStatement(CHECK_OK);