Dart: Better handle leak checks. close() is async.

examples/dart/wget/main.dart

Index: examples/dart/wget/main.dart
diff --git a/examples/dart/wget/main.dart b/examples/dart/wget/main.dart
index c43211fcf76cf7aab2f0bcf627fb81e6de65c523..a29c2c012be84c62b773d0f7cd34b925603d1199 100644
--- a/examples/dart/wget/main.dart
+++ b/examples/dart/wget/main.dart
@@ -35,15 +35,16 @@ class WGet extends Application {
     print(">>> EOF <<<");
 
     _closeProxies();
-    close();
+    await close();
+    assert(MojoHandle.reportLeakedHandles());
   }
 
   Future<ByteData> _getUrl(String url) async {
     _initProxiesIfNeeded();
 
     var urlRequest = new UrlRequest()
-        ..url = url
-        ..autoFollowRedirects = true;
+      ..url = url
+      ..autoFollowRedirects = true;
 
     var urlResponse = await _urlLoader.ptr.start(urlRequest);
     print(">>> Headers <<<");