Add BrowserManager and IframeTest for communicating with browser tests.

lib/src/utils.dart

Index: lib/src/utils.dart
diff --git a/lib/src/utils.dart b/lib/src/utils.dart
index 6b0939fe5ccf805bcf9ffb33e5e9d86b045eb1c1..a1199bcd583894bb94bb6406be404ab506d712ec 100644
--- a/lib/src/utils.dart
+++ b/lib/src/utils.dart
@@ -63,3 +63,13 @@ List flatten(Iterable nested) {
   helper(nested);
   return result;
 }
+
+/// Returns a sink that maps events sent to [original] using [fn].
+StreamSink mapSink(StreamSink original, fn(event)) {
+  var controller = new StreamController(sync: true);
+  controller.stream.listen(
+      (event) => original.add(fn(event)),
+      onError: (error, stackTrace) => original.addError(error, stackTrace),
+      onDone: () => original.close());
+  return controller.sink;
+}