Move DomProxyMixin to this package

lib/src/dom_proxy_mixin.dart

+// Copyright (c) 2014, 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.src.dom_proxy_mixin;
+
+import 'dart:js' as js;
+
+/// 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 DomProxyMixin {
+  js.JsObject _proxy;
+  js.JsObject get jsElement {
+    if (_proxy == null) {
+      _proxy = new js.JsObject.fromBrowserObject(this);
+    }
+    return _proxy;
+  }
+}