Add template comments; update copyright in template file in anticipation of new uses.

lib/dom/scripts/templateloader.py

 #!/usr/bin/python
 # Copyright (c) 2011, 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.
 
 # Template loader and preprocessor.
 #
 # Preprocessor language:
 #
+#   //$ Comment line removed by preprocessor
 #   $if VAR
 #   $else
 #   $endif
 #
 # VAR must be defined in the conditions dictionary.
 
 import os
 
 class TemplateLoader(object):
   """Loads template files from a path."""
@@ skipping 74 matching lines @@
 
         elif directive == '$endif':
           if not condition_stack:
             error(lineno, '$endif without $if')
           (active, seen_else) = condition_stack.pop()
 
         else:
           # Something else, like '$!MEMBERS'
           if active:
             out.append(full_line)
+      elif line.startswith('//$'):
+        pass  # Ignore pre-processor comment.
 
       else:
         if active:
           out.append(full_line)
         continue
 
     if condition_stack:
       error(len(lines), 'Unterminated $if')
 
     return ''.join(out)