[es6] implement default/optional parameters (WIP / comments)

src/preparser.cc

Index: src/preparser.cc
diff --git a/src/preparser.cc b/src/preparser.cc
index 15fca26788d56ad5a970d17bcbb834b3d796f244..5a1e5422c9015f1032ffc1e4292146f1f867cf1a 100644
--- a/src/preparser.cc
+++ b/src/preparser.cc
@@ -923,9 +923,11 @@ PreParser::Expression PreParser::ParseFunctionLiteral(
        arity_restriction != FunctionLiteral::SETTER_ARITY);
   while (!done) {
     bool is_strict_reserved = false;
+    int rest_pos;
     is_rest = peek() == Token::ELLIPSIS && allow_harmony_rest_params();
     if (is_rest) {
       Consume(Token::ELLIPSIS);
+      rest_pos = position();
     }
 
     Identifier param_name =
@@ -943,6 +945,19 @@ PreParser::Expression PreParser::ParseFunctionLiteral(
       dupe_error_loc = scanner()->location();
     }
 
+    if (peek() == Token::ASSIGN) {
+      Consume(Token::ASSIGN);
+      static const bool accept_IN = true;
+      ExpressionT defaultValue = ParseAssignmentExpression(accept_IN, CHECK_OK);
+      if (is_rest) {
+        ReportMessageAt(Scanner::Location(rest_pos, scanner()->location().end_pos), "rest_param_default");
+        *ok = false;
+        return Expression::Default();
+      }
+      // Default value is not needed in pre-parser.
+      USE(defaultValue);
+    }
+
     if (arity_restriction == FunctionLiteral::SETTER_ARITY) break;
     done = (peek() == Token::RPAREN);
     if (!done) {