Clarify presentation of the target in retaining paths.

runtime/observatory/lib/src/elements/unknown_ref.dart

 // Copyright (c) 2013, 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';
 import 'dart:async';
 import 'package:observatory/models.dart' as M show IsolateRef, UnknownObjectRef;
 import 'package:observatory/src/elements/helpers/rendering_scheduler.dart';
 import 'package:observatory/src/elements/helpers/tag.dart';
 import 'package:observatory/src/elements/helpers/uris.dart';
@@ skipping 34 matching lines @@
   @override
   void detached() {
     super.detached();
     _r.disable(notify: true);
     children = [];
   }
 
   void render() {
     children = [
       new AnchorElement(href: Uris.inspect(_isolate, object: _obj))
+        ..classes = ['emphasize']
         ..text = _obj.vmType
     ];
   }
 }