test.py In case we don't have any TestCases, don't block

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 33ac5120b1e21d99dd71d6eec6a24aa22118e855..1012082a09c9db18dd90d3520551334e2136240c 100644
--- a/tools/testing/dart/test_runner.dart
+++ b/tools/testing/dart/test_runner.dart
@@ -1927,6 +1927,7 @@ class TestCaseCompleter {
   final CommandQueue commandQueue;
 
   Map<Command, CommandOutput> _outputs = new Map<Command, CommandOutput>();
+  bool _closed = false;
   StreamController<TestCase> _controller = new StreamController<TestCase>();
 
   TestCaseCompleter(this.graph, this.enqueuer, this.commandQueue) {
@@ -1946,11 +1947,24 @@ class TestCaseCompleter {
             assert(COMPLETED_STATES.contains(event.to));
             _completeTestCasesIfPossible(event.node.userData);
 
-            if (graph.isSealed && enqueuer.remainingTestCases.isEmpty) {
+            if (!_closed &&
+                graph.isSealed &&
+                enqueuer.remainingTestCases.isEmpty) {
               _controller.close();
+              _closed = true;
             }
           }
     });
+
+    // Listen also for GraphSealedEvent's. If there is not a single node in the
+    // graph, we still want to finish after the graph was sealed.
+    eventCondition((event) => event is dgraph.GraphSealedEvent)
+        .listen((dgraph.GraphSealedEvent event) {
+          if (!_closed && enqueuer.remainingTestCases.isEmpty) {
+            _controller.close();
+            _closed = true;
+          }
+    });
   }
 
   Stream<TestCase> get finishedTestCases => _controller.stream;