Move DomProxyMixin to this package

lib/custom_element_proxy.dart

Index: lib/custom_element_proxy.dart
diff --git a/lib/custom_element_proxy.dart b/lib/custom_element_proxy.dart
index 01e63d5c3e7618d1c1ccdaddfc67d1d3ac8b5943..4f57097c6d20e50d2e18103e39df0eb7a94afeae 100644
--- a/lib/custom_element_proxy.dart
+++ b/lib/custom_element_proxy.dart
@@ -3,6 +3,7 @@
 // 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';
 
@@ -22,3 +23,17 @@ class CustomElementProxy implements Initializer<Type> {
     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;
+  }
+}