Dart: Simplifies the handle watcher. Various cleanups and bugfixes.

mojo/dart/test/simple_handle_watcher_test.dart

Index: mojo/dart/test/simple_handle_watcher_test.dart
diff --git a/mojo/dart/test/simple_handle_watcher_test.dart b/mojo/dart/test/simple_handle_watcher_test.dart
index 9e7bfcd4aefaf091158ef187253be0aa3581e802..6ce9796b92a0c5e47b9d0ddb98ac2c65acc6c0e5 100644
--- a/mojo/dart/test/simple_handle_watcher_test.dart
+++ b/mojo/dart/test/simple_handle_watcher_test.dart
@@ -17,13 +17,13 @@ main() {
   var completer = new Completer();
   int numEvents = 0;
 
-  handle.enableWriteEvents();
   handle.listen((_) {
     numEvents++;
     handle.close();
   }, onDone: () {
     completer.complete(numEvents);
   });
+  handle.enableWriteEvents();
 
   completer.future.then((int numEvents) {
     assert(numEvents ==1);