Get rid of System classes for backend generators.

lib/html/scripts/dartgenerator.py

Index: lib/html/scripts/dartgenerator.py
diff --git a/lib/html/scripts/dartgenerator.py b/lib/html/scripts/dartgenerator.py
index e15ebadc229a329f2edbfcaf1ba85669ddf3b22b..78ed8bc0af58709c8d0c6665e876e57f7ba71bcb 100755
--- a/lib/html/scripts/dartgenerator.py
+++ b/lib/html/scripts/dartgenerator.py
@@ -226,13 +226,8 @@ class DartGenerator(object):
             interface_name, auxiliary_file))
         continue
 
-      if 'Callback' in interface.ext_attrs:
-        handlers = [op for op in interface.operations if op.id == 'handleEvent']
-        info = AnalyzeOperation(interface, handlers)
-        system.ProcessCallback(interface, info)
-      else:
-        _logger.info('Generating %s' % interface.id)
-        system.ProcessInterface(interface)
+      _logger.info('Generating %s' % interface.id)
+      system.ProcessInterface(interface)
 
   def _PreOrderInterfaces(self, interfaces):
     """Returns the interfaces in pre-order, i.e. parents first."""