Fix flaky error on buildbot due to virtual X server xvfb.

tools/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 143 matching lines @@
     if (exitCode != 0 && !hasCrashed) return true;
 
     // Browser tests fail unless stdout contains
     // 'Content-Type: text/plain\nPASS'.
     if (testCase is !BrowserTestCase) return false;
     String previous_line = '';
     for (String line in stdout) {
       if (line == 'PASS' && previous_line == 'Content-Type: text/plain') {
         return false;
       }
+      // If the browser test failed, it may have been because DumpRenderTree
+      // and the virtual framebuffer X server didn't hook up.  So return false
+      // if we get the X server error.  Issue dart:1135 is filed.
+      if (line.contains('Gtk-WARNING **: cannot open display: :99')) {
+        return false;
+      }
       previous_line = line;
     }
     return true;
   }
 
   // Reverse result of a negative test.
   bool get hasFailed() => (testCase.isNegative ? !didFail : didFail);
 }
 
 
@@ skipping 320 matching lines @@
           test.displayName != 'dartc/junit_tests') {
         _ensureDartcBatchRunnersStarted(test.executablePath);
         _getDartcBatchRunnerProcess().startTest(test);
       } else {
         new RunningProcess(test).start();
       }
       _numProcesses++;
     }
   }
 }