Make it possible for sky-element to register any tag name

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // WARNING: Do not edit - generated code.
 part of sky.core;
 
 {% macro args_macro(args) -%}
     {%- for arg in args -%}
         {%- if arg.is_optional and (loop.first or not args[arg.index-1].is_optional) -%}
         {{ '{' if arg.is_named else '[' }}
         {%- endif -%}
         {{ arg.dart_type }} {{ arg.name }}
         {%- if arg.is_optional %} {{ ':' if arg.is_named else '='}} {{ arg.dart_default_value }}
         {#- TODO(eseidel): This does not support having both optional and named arguments! -#}
         {%- if loop.last -%}{{ '}' if arg.is_named else ']' }}{%- endif -%}
         {%- endif -%}
         {%- if not loop.last %}, {% endif %}
     {%- endfor -%}
 {%- endmacro -%}
 
-{% if not constructors %}abstract {% endif -%}
+{% if not constructors and not custom_constructors %}abstract {% endif -%}
 class {{interface_name}} extends {{ parent_interface if parent_interface else 'NativeFieldWrapperClass2' }} {
     // Constructors
 {# TODO(eseidel): We only ever have one constructor. #}
-{%- for constructor in constructors %}
+{%- for constructor in constructors + custom_constructors %}
     void _constructor(
     {%- for arg in constructor.arguments -%}
         {{ arg.dart_type }} {{ arg.name }}{% if not loop.last %}, {% endif %}
     {%- endfor -%}
     ) native "{{interface_name}}_constructorCallback";
     {{interface_name}}({{ args_macro(constructor.arguments) }}) { _constructor(
     {%- for arg in constructor.arguments -%}
         {{ arg.name }}{% if not loop.last %}, {% endif %}
     {%- endfor -%}
     ); }
@@ skipping 13 matching lines @@
 {% endfor %}
 
     // Operators
 {% if named_property_getter %}
     String operator[](String name) native "{{interface_name}}___getter___Callback";
 {% endif %}
 {% if named_property_setter %}
     void operator[]=(String name, String value) native "{{interface_name}}___setter___Callback";
 {% endif %}
 }