Use dartfmt on Observatory code

runtime/observatory/lib/src/elements/helpers/nav_bar.dart

 // Copyright (c) 2016, 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.
 
 import 'dart:html';
 
 Element navBar(Iterable<Element> content) {
   assert(content != null);
   return document.createElement('nav')
     ..classes = ['nav-bar']
     ..children = [
-      new UListElement()
-        ..children = content,
+      new UListElement()..children = content,
     ];
 }