Use a better error message when $!MEMBERS isn't used in a template file.

lib/dom/scripts/systemhtml.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 system to generate
 Dart:html APIs from the IDL database."""
 
 from systemfrog import *
 from systeminterface import *
@@ skipping 778 matching lines @@
 
     self._members_emitter = self._dart_code.Emit(
         self._template,
         #class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
         #$!MEMBERS
         #}
         CLASSNAME=self._class_name,
         EXTENDS=extends,
         IMPLEMENTS=' implements ' + ', '.join(implements),
         NATIVESPEC=' native "' + native_spec + '"')
+    if self._members_emitter == None:
+      raise Exception("Class %s doesn't use the $!MEMBERS variable" %
+                      self._class_name)
 
     # Emit a factory provider class for the constructor.
     constructor_info = AnalyzeConstructor(interface)
     if constructor_info:
       self._EmitFactoryProvider(interface_name, constructor_info)
 
     emit_events, events = self._shared.GetEventAttributes(self._interface)
     if not emit_events:
       return
     elif events:
@@ skipping 908 matching lines @@
     # dispatch has removed f(X), leaving only f(Y), but there is no guarantee
     # that Y = Z-X, so we need to check for Y.
     true_code = emitter.Emit(
         '$(INDENT)if ($COND) {\n'
         '$!TRUE'
         '$(INDENT)}\n',
         COND=test, INDENT=indent)
     self.GenerateDispatch(
         true_code, info, indent + '  ', position + 1, positive)
     return True