Factor formal argument parsing into ParserBase

src/scopes.h

Index: src/scopes.h
diff --git a/src/scopes.h b/src/scopes.h
index 7b372d9643fe4c1c07892139c6ecf1937ed1257e..409fa2f2523f58c492670ca4b3188d6e31167dd5 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);
+                             bool is_rest, bool* is_duplicate);
 
   // Declare a local variable in this scope. If the variable has been
   // declared before, the previously declared variable is returned.