replace const Duration() with Duration.ZERO (#8835)

packages/flutter/test/foundation/service_extensions_test.dart

@@ -53,12 +53,12 @@ class TestServiceExtensionsBinding extends BindingBase
   void doFrame() {
     frameScheduled = false;
     if (ui.window.onBeginFrame != null)
-      ui.window.onBeginFrame(const Duration());
+      ui.window.onBeginFrame(Duration.ZERO);
   }
 
   Future<Null> flushMicrotasks() {
     final Completer<Null> completer = new Completer<Null>();
-    new Timer(const Duration(), completer.complete);
+    Timer.run(completer.complete);
     return completer.future;
   }
 }

packages/flutter/test/services/platform_channel_test.dart

@@ -101,8 +101,8 @@ void main() {
       );
       final List<dynamic> events = await channel.receiveBroadcastStream('hello').toList();
       expect(events, orderedEquals(<String>['hello1', 'hello2']));
-      await new Future<Null>.delayed(const Duration());
+      await new Future<Null>.delayed(Duration.ZERO);
       expect(cancelled, isTrue);
     });
   });
-}
\ No newline at end of file
+}

packages/flutter/test/widgets/async_test.dart

@@ -255,7 +255,7 @@ void main() {
 }
 
 Future<Null> eventFiring(WidgetTester tester) async {
-  await tester.pump(const Duration());
+  await tester.pump(Duration.ZERO);
 }
 
 class StringCollector extends StreamBuilderBase<String, List<String>> {