Move DomProxyMixin to this package

lib/custom_element_proxy.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 library web_components.custom_element_proxy;
 
+import 'dart:js' as js;
 import 'package:initialize/initialize.dart';
 import 'interop.dart';
 
 /// Annotation for a dart class which proxies a javascript custom element.
 /// This will not work unless `interop_support.js` is loaded.
 // TODO(jakemac): Add an @HtmlImport here to a new file which includes
 // `interop_support.js`. We will need to point everything else at that html file
 // as well for deduplication purposes (could even just copy it in as an inline
 // script so the js file no longer exists?).
 class CustomElementProxy implements Initializer<Type> {
   final String tagName;
   final String extendsTag;
 
   const CustomElementProxy(this.tagName, {this.extendsTag});
 
   void initialize(Type t) {
     registerDartType(tagName, t, extendsTag: extendsTag);
   }
 }
+
+/// A simple mixin to make it easier to interoperate with the Javascript API of
+/// a browser object. This is mainly used by classes that expose a Dart API for
+/// Javascript custom elements.
+class CustomElementProxyMixin {
+  js.JsObject _proxy;
+
+  js.JsObject get jsElement {
+    if (_proxy == null) {
+      _proxy = new js.JsObject.fromBrowserObject(this);
+    }
+    return _proxy;
+  }
+}