Only print the failure summary header if there were failures

tools/testing/dart/test_progress.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.
 
 library test_progress;
 
 import "dart:io";
 import "dart:io" as io;
 import "http_server.dart" as http_server;
 import "status_file_parser.dart";
@@ skipping 326 matching lines @@
       print('');
       if (_printSummary) {
         _failureSummary.addAll(lines);
         _failureSummary.add('');
       }
     }
   }
 
   void allDone() {
     if (_printSummary) {
-      print('\n=== Failure summary:\n');
       if (!_failureSummary.isEmpty) {
+        print('\n=== Failure summary:\n');
         for (String line in _failureSummary) {
           print(line);
         }
         print('');
       }
     }
   }
 }
 
 class ProgressIndicator extends EventListener {
@@ skipping 158 matching lines @@
         print('@@@BUILD_STEP $stepName failures@@@');
       }
       for (String line in _failureSummary) {
         print(line);
       }
       print('');
     }
     super.allDone();
   }
 }