Improve the problem description.

lib/dom/scripts/systemnative.py

 #!/usr/bin/python
 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 """This module provides shared functionality for the systems to generate
 native binding from the IDL database."""
 
 import emitter
 import os
@@ skipping 647 matching lines @@
             GenerateChecksAndCall(operation, position)
         GenerateChecksAndCall(operation, len(operation.arguments))
       body.Emit('    throw "Incorrect number or type of arguments";\n');
     else:
       operation = operations[0]
       argument_count = len(operation.arguments)
       for position, argument in list(enumerate(operation.arguments))[::-1]:
         if self._IsArgumentOptionalInWebCore(operation, argument):
           check = '%s !== _null' % argument_names[position]
           # argument_count instead of position + 1 is used here to cover one
-          # complicated case.  Consider foo(x, [Optional] y, [Optional=DefaultIsNullString] z)
+          # complicated case with the effectively optional argument in the middle.
+          # Consider foo(x, [Optional] y, [Optional=DefaultIsNullString] z)
           # (as of now it's modelled after HTMLMediaElement.webkitAddKey).
           # y is optional in WebCore, while z is not.
           # In this case, if y !== _null, we'd like to emit foo(x, y, z) invocation, not
           # foo(x, y).
           GenerateCall(operation, argument_count, [check])
           argument_count = position
       GenerateCall(operation, argument_count, [])
 
   def SecondaryContext(self, interface):
     pass
@@ skipping 269 matching lines @@
     parent_interface = _FindInHierarchy(database, parent_interface, test)
     if parent_interface:
       return parent_interface
 
 def _ToWebKitName(name):
   name = name[0].lower() + name[1:]
   name = re.sub(r'^(hTML|uRL|jS|xML|xSLT)', lambda s: s.group(1).lower(),
                 name)
   return re.sub(r'^(create|exclusive)', lambda s: 'is' + s.group(1).capitalize(),
                 name)