Sky: Add a CustomDart attribute to the IDL compiler, and use that with Canvas

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 7576a537c4b52082668a213a10438b9fcbff6df0..d0622c33d23cb7f0206be839de7314f73a3d3910 100644
--- a/sky/engine/bindings/scripts/templates/interface_dart.template
+++ b/sky/engine/bindings/scripts/templates/interface_dart.template
@@ -19,8 +19,10 @@ part of dart.sky;
     {%- endfor -%}
 {%- endmacro -%}
 
+{%- set prefix = "_" if custom_dartcode else "" -%}
 {% if not constructors and not custom_constructors %}abstract {% endif -%}
-class {{interface_name}} extends {{ parent_interface if parent_interface else 'NativeFieldWrapperClass2' }} {
+class {{prefix}}{{interface_name}} extends
+    {{ parent_interface if parent_interface else 'NativeFieldWrapperClass2' }} {
     // Constructors
 {# TODO(eseidel): We only ever have one constructor. #}
 {%- for constructor in constructors + custom_constructors %}
@@ -38,15 +40,15 @@ class {{interface_name}} extends {{ parent_interface if parent_interface else 'N
 
     // Attributes
 {% for attribute in attributes %}
-    {{ attribute.dart_type }} get {{ attribute.name }} native "{{interface_name}}_{{ attribute.name }}_Getter";
+    {{ attribute.dart_type }} get {{prefix}}{{ attribute.name }} native "{{interface_name}}_{{ attribute.name }}_Getter";
 {% if not attribute.is_read_only %}
-    void set {{ attribute.name }}({{ attribute.dart_type }} value) native "{{interface_name}}_{{ attribute.name }}_Setter";
+    void set {{prefix}}{{ attribute.name }}({{ attribute.dart_type }} value) native "{{interface_name}}_{{ attribute.name }}_Setter";
 {% endif %}
 {% endfor %}
 
     // Methods
 {% for method in methods %}
-    {{method.dart_type}} {{method.name}}({{ args_macro(method.arguments)}}) native "{{interface_name}}_{{ method.name }}_Callback";
+    {{method.dart_type}} {{prefix}}{{method.name}}({{ args_macro(method.arguments)}}) native "{{interface_name}}_{{ method.name }}_Callback";
 {% endfor %}
 
     // Operators
@@ -57,3 +59,5 @@ class {{interface_name}} extends {{ parent_interface if parent_interface else 'N
     void operator[]=(String name, String value) native "{{interface_name}}___setter___Callback";
 {% endif %}
 }
+
+{{custom_dartcode}}