tools: fix break in test_progress

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:async";
 import "dart:io";
 import "dart:io" as io;
 import "dart:convert" show JSON;
@@ skipping 292 matching lines @@
         }
       }
     }
   }
 }
 
 
 class SummaryPrinter extends EventListener {
   final bool jsonOnly;
 
-  SummaryPrinter({this.jsonOnly});
+  SummaryPrinter({bool jsonOnly})
+      : jsonOnly = (jsonOnly == null) ? false : jsonOnly;
 
   void allTestsKnown() {
     if (jsonOnly) {
       print("JSON:");
       print(JSON.encode(summaryReport.values));
     } else {
       summaryReport.printReport();
     }
   }
 }
@@ skipping 345 matching lines @@
       return new VerboseProgressIndicator(startTime);
     case 'status':
       return new ProgressIndicator(startTime);
     case 'buildbot':
       return new BuildbotProgressIndicator(startTime);
     default:
       assert(false);
       break;
   }
 }