Avoid extra arity and type check on distinguishing argument

Source/bindings/scripts/v8_interface.py

 # Copyright (C) 2013 Google Inc. All rights reserved.
 # coding=utf-8
 #
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@@ skipping 596 matching lines @@
     methods = [effective_overload[0]
                for effective_overload in effective_overloads]
     if len(methods) == 1:
         # If only one method with a given length, no test needed
         yield ('true', methods[0])
         return
 
     # 6. If there is more than one entry in S, then set d to be the
     # distinguishing argument index for the entries of S.
     index = distinguishing_argument_index(effective_overloads)
+    for method in methods:
+        method['distinguishing_argument_index'] = index
+
     # (7-9 are for handling |undefined| values for optional arguments before
     # the distinguishing argument (as “missing”), so you can specify only some
     # optional arguments. We don’t support this, so we skip these steps.)
     # 10. If i = d, then:
     # (d is the distinguishing argument index)
     # 1. Let V be argi.
     #     Note: This is the argument that will be used to resolve which
     #           overload is selected.
     cpp_value = 'info[%s]' % index
 
@@ skipping 492 matching lines @@
         deleter = next(
             method
             for method in interface.operations
             if ('deleter' in method.specials and
                 len(method.arguments) == 1 and
                 str(method.arguments[0].idl_type) == 'DOMString'))
     except StopIteration:
         return None
 
     return property_deleter(deleter)