Fix all dartanalyzer errors in dart:sky except the Native ones.

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;
+part of dart.sky;
 
 {% 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 -%}
@@ skipping 33 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 %}
 }