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

src/scopes.h

Index: src/scopes.h
diff --git a/src/scopes.h b/src/scopes.h
index 7b372d9643fe4c1c07892139c6ecf1937ed1257e..5ed8af1d082c4c0a07497b53f06db51cd5b3d789 100644
--- a/src/scopes.h
+++ b/src/scopes.h
@@ -125,7 +125,7 @@ class Scope: public ZoneObject {
   // parameters the rightmost one 'wins'.  However, the implementation
   // expects all parameters to be declared and from left to right.
   Variable* DeclareParameter(const AstRawString* name, VariableMode mode,
-                             bool is_rest = false);
+                             ParameterKind kind = ParameterKind::Normal);
 
   // Declare a local variable in this scope. If the variable has been
   // declared before, the previously declared variable is returned.