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

tools/testing/dart/test_runner.dart

Index: tools/testing/dart/test_runner.dart
diff --git a/tools/testing/dart/test_runner.dart b/tools/testing/dart/test_runner.dart
index 50721c44ccce5ff0973b93c084c3eec6250e9010..370022f073c0a9601b26b2a025dd5793a9df1aa5 100644
--- a/tools/testing/dart/test_runner.dart
+++ b/tools/testing/dart/test_runner.dart
@@ -161,6 +161,12 @@ class TestOutput {
       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;