Revert "Revert "Start of dartium custom element lifecycle event support.""

tools/dom/scripts/systemnative.py

Index: tools/dom/scripts/systemnative.py
diff --git a/tools/dom/scripts/systemnative.py b/tools/dom/scripts/systemnative.py
index 038825c60331ff7c517d43e90bc72947f67d7cb0..8dbe0e224ec7e1724b57227cbb913930aee21c5a 100644
--- a/tools/dom/scripts/systemnative.py
+++ b/tools/dom/scripts/systemnative.py
@@ -789,13 +789,13 @@ class DartiumBackend(HtmlDartGenerator):
     if 'Reflect' in ext_attrs:
       cpp_arguments = [self._GenerateWebCoreReflectionAttributeName(node)]
 
-    if return_type_is_nullable:
-      cpp_arguments = ['isNull']
-
     if ext_attrs.get('CustomElementCallbacks') == 'Enable':
       self._cpp_impl_includes.add('"core/dom/CustomElementCallbackDispatcher.h"')
       needs_custom_element_callbacks = True
 
+    if return_type_is_nullable:
+      cpp_arguments = ['isNull']
+
     v8EnabledPerContext = ext_attrs.get('synthesizedV8EnabledPerContext', ext_attrs.get('V8EnabledPerContext'))
     v8EnabledAtRuntime = ext_attrs.get('synthesizedV8EnabledAtRuntime', ext_attrs.get('V8EnabledAtRuntime'))
     assert(not (v8EnabledPerContext and v8EnabledAtRuntime))