Change the way we provide custom dart code for IDL bindings.

sky/engine/bindings/scripts/templates/interface_dart.template

Index: sky/engine/bindings/scripts/templates/interface_dart.template
diff --git a/sky/engine/bindings/scripts/templates/interface_dart.template b/sky/engine/bindings/scripts/templates/interface_dart.template
index 42504c19cf2b882be9a52f7829b09b0f644db18d..dc421b72cef2b88c4cbfb5f9ab39858e8d88726f 100644
--- a/sky/engine/bindings/scripts/templates/interface_dart.template
+++ b/sky/engine/bindings/scripts/templates/interface_dart.template
@@ -19,7 +19,7 @@ part of dart.sky;
     {%- endfor -%}
 {%- endmacro -%}
 
-{%- set prefix = "_" if custom_dartcode else "" -%}
+{%- set prefix = "_" if private_dart else "" -%}
 {% if not constructors and not custom_constructors %}abstract {% endif -%}
 class {{prefix}}{{interface_name}} extends
     {{ parent_interface if parent_interface else 'NativeFieldWrapperClass2' }} {
@@ -59,5 +59,3 @@ class {{prefix}}{{interface_name}} extends
     void operator[]=(String name, String value) native "{{interface_name}}___setter___Callback";
 {% endif %}
 }
-
-{{custom_dartcode}}