Gracefully handle tests that print.

lib/src/backend/invoker.dart

Index: lib/src/backend/invoker.dart
diff --git a/lib/src/backend/invoker.dart b/lib/src/backend/invoker.dart
index 53d9ad0b27a0bbca6d2f773568236eae8763ba4b..fe4d0fbac50223e2719a0ae6e0e666f6a758fdb3 100644
--- a/lib/src/backend/invoker.dart
+++ b/lib/src/backend/invoker.dart
@@ -178,7 +178,11 @@ class Invoker {
           // non-microtask events.
           Timer.run(_controller.completer.complete);
         });
-      }, zoneValues: {#unittest.invoker: this}, onError: handleError);
+      },
+          zoneSpecification: new ZoneSpecification(
+              print: (self, parent, zone, line) => _controller.print(line)),
+          zoneValues: {#unittest.invoker: this},
+          onError: handleError);
     });
   }
 }