Factor formal argument parsing into ParserBase

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 5d8ff65627a1dafa79bbf5cd265abe41a9d9861d..db0f9214d4bc65a32676819462530e251cd73b40 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -181,7 +181,9 @@ var kMessages = {
   array_not_subclassable:        ["Subclassing Arrays is not currently supported."],
   for_in_loop_initializer:       ["for-in loop variable declaration may not have an initializer."],
   for_of_loop_initializer:       ["for-of loop variable declaration may not have an initializer."],
-  for_inof_loop_multi_bindings:  ["Invalid left-hand side in ", "%0", " loop: Must have a single binding."]
+  for_inof_loop_multi_bindings:  ["Invalid left-hand side in ", "%0", " loop: Must have a single binding."],
+  bad_getter_arity:              ["Getter must not have any formal parameters."],
+  bad_setter_arity:              ["Setter must have exactly one formal parameter."]
 };