Address comment from https://chromiumcodereview.appspot.com/9150027:

testing/dart/test_runner.dart

 // Copyright (c) 2011, 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_runner");
 
 #import("status_file_parser.dart");
 #import("test_progress.dart");
 #import("test_suite.dart");
 
@@ skipping 59 matching lines @@
       suffixSplit.forEach((e) {
         if (!e.isEmpty()) newArguments.add(e);
       });
       arguments = newArguments;
     }
   }
 
   int get timeout() => configuration['timeout'];
 
   String get configurationString() =>
-      configuration['mode'] + configuration['arch'];
+      "${configuration['mode']}_${configuration['arch']}";
 
   void completed() { completedHandler(this); }
 }
 
 
 /**
  * BrowserTestCase has an extra compilation command that is run by
  * RunningProcess.start(), and it checks conditions on the test output
  * in TestOutput.didFail().
  */
@@ skipping 452 matching lines @@
           test.displayName != 'dartc/junit_tests') {
         _ensureDartcBatchRunnersStarted(test.executablePath);
         _getDartcBatchRunnerProcess().startTest(test);
       } else {
         new RunningProcess(test).start();
       }
       _numProcesses++;
     }
   }
 }