Added json result of test output to output debug directory.

tools/testing/dart/test_configurations.dart

Index: tools/testing/dart/test_configurations.dart
diff --git a/tools/testing/dart/test_configurations.dart b/tools/testing/dart/test_configurations.dart
index 7b9be0e1e9490b252afc3e50297c8d8ab753137a..dfae674b264f9f21c07642fe6d81df08b9b39f55 100644
--- a/tools/testing/dart/test_configurations.dart
+++ b/tools/testing/dart/test_configurations.dart
@@ -233,6 +233,10 @@ Future testConfigurations(List<Configuration> configurations) async {
     eventListener.add(new TestOutcomeLogWriter());
   }
 
+  if (firstConf.writeResultLog) {
+    eventListener.add(new ResultLogWriter());
+  }
+
   if (firstConf.copyCoreDumps) {
     eventListener.add(new UnexpectedCrashLogger());
   }