Fixes for html report

lib/src/report/html_reporter.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 dev_compiler.src.html_reporter;
 
 import 'dart:collection' show LinkedHashSet;
 import 'dart:convert' show HTML_ESCAPE;
 import 'dart:io';
 
@@ skipping 75 matching lines @@
             .toList();
         summaries.add(
             new SummaryInfo('Package: code', name, 'package:${name}', issues));
       }
     }
 
     // dart: code
     keys = result.system.keys.toList()..sort();
     for (String name in keys) {
       LibrarySummary summary = result.system[name];
-      summaries.add(new SummaryInfo(
-          'Dart: code', name, 'dart:${name}', summary.messages));
+      if (summary.messages.isNotEmpty) {
+        summaries.add(new SummaryInfo(
+            'Dart: code', name, 'dart:${name}', summary.messages));
+      }
     }
 
     // Loose files
     if (result.loose.isNotEmpty) {
       List<MessageSummary> issues = result.loose.values
           .expand((IndividualSummary summary) => summary.messages)
           .toList();
       summaries.add(new SummaryInfo('Files', 'files', 'files', issues));
     }
 
@@ skipping 415 matching lines @@
 }
 
 .blob-code-expandable {
   padding-top: 4px;
   padding-bottom: 4px;
   background-color: #f4f7fb;
   border-width: 1px 0;
   border-bottom-right-radius: 3px;
 }
 ''';