Clarify presentation of the target in retaining paths.

runtime/observatory/lib/src/elements/objectpool_view.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 objectpool_view;
 
 import 'dart:async';
 import 'dart:html';
 import 'package:observatory/models.dart' as M;
 import 'package:observatory/src/elements/context_ref.dart';
@@ skipping 154 matching lines @@
         return [
           new SpanElement()
             ..text = 'Immediate 0x${entry.asInteger.toRadixString(16)}'
         ];
       case M.ObjectPoolEntryKind.nativeEntry:
         return [
           new SpanElement()
             ..text = 'NativeEntry 0x${entry.asInteger.toRadixString(16)}'
         ];
     }
-    throw new Exception('Unkown ObjectPoolEntryKind (${entry.kind})');
+    throw new Exception('Unknown ObjectPoolEntryKind (${entry.kind})');
   }
 }