Gracefully handle tests that print.

lib/src/runner/browser/iframe_listener.dart

Index: lib/src/runner/browser/iframe_listener.dart
diff --git a/lib/src/runner/browser/iframe_listener.dart b/lib/src/runner/browser/iframe_listener.dart
index 49ab7d96be576288dd6af5eb22d9eda10bd114b4..9501959bc0fca72a0a7d7145ddfddf69806c477e 100644
--- a/lib/src/runner/browser/iframe_listener.dart
+++ b/lib/src/runner/browser/iframe_listener.dart
@@ -159,6 +159,11 @@ class IframeListener {
       });
     });
 
+    liveTest.onPrint.listen((line) {
+      print(line);
+      channel.sink.add({"type": "print", "line": line});
+    });
+
     liveTest.run().then((_) => channel.sink.add({"type": "complete"}));
   }
 }