use Duration.zero (#23218)

packages/flutter/lib/src/cupertino/date_picker.dart

@@ -958,13 +958,13 @@ class CupertinoTimerPicker extends StatefulWidget {
   /// positive integer factor of 60.
   CupertinoTimerPicker({
     this.mode = CupertinoTimerPickerMode.hms,
-    this.initialTimerDuration = const Duration(),
+    this.initialTimerDuration = Duration.zero,
     this.minuteInterval = 1,
     this.secondInterval = 1,
     @required this.onTimerDurationChanged,
   }) : assert(mode != null),
        assert(onTimerDurationChanged != null),
-       assert(initialTimerDuration >= const Duration(seconds: 0)),
+       assert(initialTimerDuration >= Duration.zero),
        assert(initialTimerDuration < const Duration(days: 1)),
        assert(minuteInterval > 0 && 60 % minuteInterval == 0),
        assert(secondInterval > 0 && 60 % secondInterval == 0),

packages/flutter/lib/src/scheduler/ticker.dart

@@ -299,7 +299,7 @@ class Ticker {
       // We intentionally don't null out _startTime. This means that if start()
       // was ever called, the object is now in a bogus state. This weakly helps
       // catch cases of use-after-dispose.
-      _startTime = const Duration();
+      _startTime = Duration.zero;
       return true;
     }());
   }

packages/flutter_test/lib/src/binding.dart

@@ -773,7 +773,7 @@ class AutomatedTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
   @override
   Future<void> idle() {
     final Future<void> result = super.idle();
-    _currentFakeAsync.elapse(const Duration());
+    _currentFakeAsync.elapse(Duration.zero);
     return result;
   }