Move DomProxyMixin to this package

lib/build/mirrors_remover.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.build.mirrors_remover;
 
 import 'dart:async';
 import 'package:barback/barback.dart';
 
 /// Removes `mirror_initializer.dart` and replaces it with
 /// `static_initializer.dart`.
 class MirrorsRemoverTransformer extends Transformer {
   MirrorsRemoverTransformer();
   MirrorsRemoverTransformer.asPlugin(BarbackSettings settings);
 
   bool isPrimary(AssetId id) => id.path == 'lib/src/init.dart';
 
   @override
   Future apply(Transform transform) async {
     String source = await transform.primaryInput.readAsString();
-    source = source.replaceAll(
-        'mirror_initializer.dart', 'static_initializer.dart');
-    transform.addOutput(
-        new Asset.fromString(transform.primaryInput.id, source));
+    source =
+        source.replaceAll('mirror_initializer.dart', 'static_initializer.dart');
+    transform
+        .addOutput(new Asset.fromString(transform.primaryInput.id, source));
   }
 }