Use void as type argument instead of Null where appropriate

examples/stocks/lib/stock_settings.dart

@@ -85,7 +85,7 @@ class StockSettingsState extends State<StockSettings> {
               ),
             ]
           )
-        ).then<Null>(_handleOptimismChanged);
+        ).then<void>(_handleOptimismChanged);
         break;
     }
   }

packages/flutter/lib/src/gestures/long_press.dart

@@ -26,7 +26,7 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
   void didExceedDeadline() {
     resolve(GestureDisposition.accepted);
     if (onLongPress != null)
-      invokeCallback<Null>('onLongPress', onLongPress); // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+      invokeCallback<void>('onLongPress', onLongPress);
   }
 
   @override

packages/flutter/lib/src/gestures/monodrag.dart

@@ -120,7 +120,7 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
       _pendingDragOffset = Offset.zero;
       _lastPendingEventTimestamp = event.timeStamp;
       if (onDown != null)
-        invokeCallback<Null>('onDown', () => onDown(new DragDownDetails(globalPosition: _initialPosition))); // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+        invokeCallback<void>('onDown', () => onDown(new DragDownDetails(globalPosition: _initialPosition)));
     } else if (_state == _DragState.accepted) {
       resolve(GestureDisposition.accepted);
     }
@@ -140,7 +140,7 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
       final Offset delta = event.delta;
       if (_state == _DragState.accepted) {
         if (onUpdate != null) {
-          invokeCallback<Null>('onUpdate', () => onUpdate(new DragUpdateDetails( // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+          invokeCallback<void>('onUpdate', () => onUpdate(new DragUpdateDetails(
             sourceTimeStamp: event.timeStamp,
             delta: _getDeltaForDetails(delta),
             primaryDelta: _getPrimaryValueFromOffset(delta),
@@ -166,13 +166,13 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
       _pendingDragOffset = Offset.zero;
       _lastPendingEventTimestamp = null;
       if (onStart != null) {
-        invokeCallback<Null>('onStart', () => onStart(new DragStartDetails( // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+        invokeCallback<void>('onStart', () => onStart(new DragStartDetails(
           sourceTimeStamp: timestamp,
           globalPosition: _initialPosition,
         )));
       }
       if (delta != Offset.zero && onUpdate != null) {
-        invokeCallback<Null>('onUpdate', () => onUpdate(new DragUpdateDetails( // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+        invokeCallback<void>('onUpdate', () => onUpdate(new DragUpdateDetails(
           sourceTimeStamp: timestamp,
           delta: _getDeltaForDetails(delta),
           primaryDelta: _getPrimaryValueFromOffset(delta),
@@ -193,7 +193,7 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
       resolve(GestureDisposition.rejected);
       _state = _DragState.ready;
       if (onCancel != null)
-        invokeCallback<Null>('onCancel', onCancel); // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+        invokeCallback<void>('onCancel', onCancel);
       return;
     }
     final bool wasAccepted = (_state == _DragState.accepted);
@@ -206,14 +206,14 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
       if (estimate != null && _isFlingGesture(estimate)) {
         final Velocity velocity = new Velocity(pixelsPerSecond: estimate.pixelsPerSecond)
           .clampMagnitude(minFlingVelocity ?? kMinFlingVelocity, maxFlingVelocity ?? kMaxFlingVelocity);
-        invokeCallback<Null>('onEnd', () => onEnd(new DragEndDetails( // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+        invokeCallback<void>('onEnd', () => onEnd(new DragEndDetails(
           velocity: velocity,
           primaryVelocity: _getPrimaryValueFromOffset(velocity.pixelsPerSecond),
         )), debugReport: () {
           return '$estimate; fling at $velocity.';
         });
       } else {
-        invokeCallback<Null>('onEnd', () => onEnd(new DragEndDetails( // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+        invokeCallback<void>('onEnd', () => onEnd(new DragEndDetails(
           velocity: Velocity.zero,
           primaryVelocity: 0.0,
         )), debugReport: () {

packages/flutter/lib/src/gestures/multitap.dart

@@ -194,7 +194,7 @@ class DoubleTapGestureRecognizer extends GestureRecognizer {
     _freezeTracker(tracker);
     _trackers.remove(tracker.pointer);
     if (onDoubleTap != null)
-      invokeCallback<Null>('onDoubleTap', onDoubleTap); // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+      invokeCallback<void>('onDoubleTap', onDoubleTap);
     _reset();
   }
 
@@ -353,7 +353,7 @@ class MultiTapGestureRecognizer extends GestureRecognizer {
       longTapDelay: longTapDelay
     );
     if (onTapDown != null)
-      invokeCallback<Null>('onTapDown', () => onTapDown(event.pointer, new TapDownDetails(globalPosition: event.position))); // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+      invokeCallback<void>('onTapDown', () => onTapDown(event.pointer, new TapDownDetails(globalPosition: event.position)));
   }
 
   @override
@@ -373,22 +373,22 @@ class MultiTapGestureRecognizer extends GestureRecognizer {
     assert(_gestureMap.containsKey(pointer));
     _gestureMap.remove(pointer);
     if (onTapCancel != null)
-      invokeCallback<Null>('onTapCancel', () => onTapCancel(pointer)); // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+      invokeCallback<void>('onTapCancel', () => onTapCancel(pointer));
   }
 
   void _dispatchTap(int pointer, Offset globalPosition) {
     assert(_gestureMap.containsKey(pointer));
     _gestureMap.remove(pointer);
     if (onTapUp != null)
-      invokeCallback<Null>('onTapUp', () => onTapUp(pointer, new TapUpDetails(globalPosition: globalPosition))); // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+      invokeCallback<void>('onTapUp', () => onTapUp(pointer, new TapUpDetails(globalPosition: globalPosition)));
     if (onTap != null)
-      invokeCallback<Null>('onTap', () => onTap(pointer)); // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+      invokeCallback<void>('onTap', () => onTap(pointer));
   }
 
   void _dispatchLongTap(int pointer, Offset lastPosition) {
     assert(_gestureMap.containsKey(pointer));
     if (onLongTapDown != null)
-      invokeCallback<Null>('onLongTapDown', () => onLongTapDown(pointer, new TapDownDetails(globalPosition: lastPosition))); // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+      invokeCallback<void>('onLongTapDown', () => onLongTapDown(pointer, new TapDownDetails(globalPosition: lastPosition)));
   }
 
   @override

packages/flutter/lib/src/gestures/scale.dart

@@ -200,9 +200,9 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
           final Offset pixelsPerSecond = velocity.pixelsPerSecond;
           if (pixelsPerSecond.distanceSquared > kMaxFlingVelocity * kMaxFlingVelocity)
             velocity = new Velocity(pixelsPerSecond: (pixelsPerSecond / pixelsPerSecond.distance) * kMaxFlingVelocity);
-          invokeCallback<Null>('onEnd', () => onEnd(new ScaleEndDetails(velocity: velocity))); // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+          invokeCallback<void>('onEnd', () => onEnd(new ScaleEndDetails(velocity: velocity)));
         } else {
-          invokeCallback<Null>('onEnd', () => onEnd(new ScaleEndDetails(velocity: Velocity.zero))); // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+          invokeCallback<void>('onEnd', () => onEnd(new ScaleEndDetails(velocity: Velocity.zero)));
         }
       }
       _state = _ScaleState.accepted;
@@ -230,13 +230,13 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
     }
 
     if (_state == _ScaleState.started && onUpdate != null)
-      invokeCallback<Null>('onUpdate', () => onUpdate(new ScaleUpdateDetails(scale: _scaleFactor, focalPoint: _currentFocalPoint))); // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+      invokeCallback<void>('onUpdate', () => onUpdate(new ScaleUpdateDetails(scale: _scaleFactor, focalPoint: _currentFocalPoint)));
   }
 
   void _dispatchOnStartCallbackIfNeeded() {
     assert(_state == _ScaleState.started);
     if (onStart != null)
-      invokeCallback<Null>('onStart', () => onStart(new ScaleStartDetails(focalPoint: _currentFocalPoint))); // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
+      invokeCallback<void>('onStart', () => onStart(new ScaleStartDetails(focalPoint: _currentFocalPoint)));
   }
 
   @override

packages/flutter/lib/src/gestures/tap.dart

@@ -103,7 +103,7 @@ class TapGestureRecognizer extends PrimaryPointerGestureRecognizer {
       // This can happen if the superclass decides the primary pointer
       // exceeded the touch slop, or if the recognizer is disposed.
       if (onTapCancel != null)
-        invokeCallback<Null>('spontaneous onTapCancel', onTapCancel);
+        invokeCallback<void>('spontaneous onTapCancel', onTapCancel);
       _reset();
     }
     super.resolve(disposition);
@@ -131,7 +131,7 @@ class TapGestureRecognizer extends PrimaryPointerGestureRecognizer {
       // Another gesture won the arena.
       assert(state != GestureRecognizerState.possible);
       if (onTapCancel != null)
-        invokeCallback<Null>('forced onTapCancel', onTapCancel);
+        invokeCallback<void>('forced onTapCancel', onTapCancel);
       _reset();
     }
   }
@@ -139,7 +139,7 @@ class TapGestureRecognizer extends PrimaryPointerGestureRecognizer {
   void _checkDown() {
     if (!_sentTapDown) {
       if (onTapDown != null)
-        invokeCallback<Null>('onTapDown', () { onTapDown(new TapDownDetails(globalPosition: initialPosition)); });
+        invokeCallback<void>('onTapDown', () { onTapDown(new TapDownDetails(globalPosition: initialPosition)); });
       _sentTapDown = true;
     }
   }
@@ -156,9 +156,9 @@ class TapGestureRecognizer extends PrimaryPointerGestureRecognizer {
         return;
       }
       if (onTapUp != null)
-        invokeCallback<Null>('onTapUp', () { onTapUp(new TapUpDetails(globalPosition: _finalPosition)); });
+        invokeCallback<void>('onTapUp', () { onTapUp(new TapUpDetails(globalPosition: _finalPosition)); });
       if (onTap != null)
-        invokeCallback<Null>('onTap', onTap);
+        invokeCallback<void>('onTap', onTap);
       _reset();
     }
   }

packages/flutter/lib/src/material/dropdown.dart

@@ -578,7 +578,7 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
       barrierLabel: MaterialLocalizations.of(context).modalBarrierDismissLabel,
     );
 
-    Navigator.push(context, _dropdownRoute).then<Null>((_DropdownRouteResult<T> newValue) {
+    Navigator.push(context, _dropdownRoute).then<void>((_DropdownRouteResult<T> newValue) {
       _dropdownRoute = null;
       if (!mounted || newValue == null)
         return null;

packages/flutter/lib/src/material/expansion_tile.dart

@@ -120,7 +120,7 @@ class _ExpansionTileState extends State<ExpansionTile> with SingleTickerProvider
       if (_isExpanded)
         _controller.forward();
       else
-        _controller.reverse().then<Null>((Null value) {
+        _controller.reverse().then<void>((Null value) {
           setState(() {
             // Rebuild without widget.children.
           });

packages/flutter/lib/src/material/popup_menu.dart

@@ -813,7 +813,7 @@ class _PopupMenuButtonState<T> extends State<PopupMenuButton<T>> {
       initialValue: widget.initialValue,
       position: position,
     )
-    .then<Null>((T newValue) {
+    .then<void>((T newValue) {
       if (!mounted || newValue == null)
         return null;
       if (widget.onSelected != null)

packages/flutter/lib/src/material/refresh_indicator.dart

@@ -322,7 +322,7 @@ class RefreshIndicatorState extends State<RefreshIndicator> with TickerProviderS
     _mode = _RefreshIndicatorMode.snap;
     _positionController
       .animateTo(1.0 / _kDragSizeFactorLimit, duration: _kIndicatorSnapDuration)
-      .then<Null>((Null value) {
+      .then<void>((Null value) {
         if (mounted && _mode == _RefreshIndicatorMode.snap) {
           assert(widget.onRefresh != null);
           setState(() {

packages/flutter/lib/src/material/scaffold.dart

@@ -678,7 +678,7 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
     if (_snackBars.isEmpty || _snackBarController.status == AnimationStatus.dismissed)
       return;
     final Completer<SnackBarClosedReason> completer = _snackBars.first._completer;
-    _snackBarController.reverse().then<Null>((Null _) {
+    _snackBarController.reverse().then<void>((Null _) {
       assert(mounted);
       if (!completer.isCompleted)
         completer.complete(reason);

packages/flutter/lib/src/painting/image_provider.dart

@@ -261,7 +261,7 @@ abstract class ImageProvider<T> {
     assert(configuration != null);
     final ImageStream stream = new ImageStream();
     T obtainedKey;
-    obtainKey(configuration).then<Null>((T key) {
+    obtainKey(configuration).then<void>((T key) {
       obtainedKey = key;
       stream.setCompleter(PaintingBinding.instance.imageCache.putIfAbsent(key, () => load(key)));
     }).catchError(

packages/flutter/lib/src/painting/image_resolution.dart

@@ -166,7 +166,7 @@ class AssetImage extends AssetBundleImageProvider {
     final AssetBundle chosenBundle = bundle ?? configuration.bundle ?? rootBundle;
     Completer<AssetBundleImageKey> completer;
     Future<AssetBundleImageKey> result;
-    chosenBundle.loadStructuredData<Map<String, List<String>>>(_kAssetManifestFileName, _manifestParser).then<Null>(
+    chosenBundle.loadStructuredData<Map<String, List<String>>>(_kAssetManifestFileName, _manifestParser).then<void>(
       (Map<String, List<String>> manifest) {
         final String chosenName = _chooseVariant(
           keyName,

packages/flutter/lib/src/painting/image_stream.dart

@@ -270,7 +270,7 @@ class OneFrameImageStreamCompleter extends ImageStreamCompleter {
   /// FlutterErrorDetails]).
   OneFrameImageStreamCompleter(Future<ImageInfo> image, { InformationCollector informationCollector })
     : assert(image != null) {
-    image.then<Null>(setImage, onError: (dynamic error, StackTrace stack) {
+    image.then<void>(setImage, onError: (dynamic error, StackTrace stack) {
       FlutterError.reportError(new FlutterErrorDetails(
         exception: error,
         stack: stack,
@@ -333,7 +333,7 @@ class MultiFrameImageStreamCompleter extends ImageStreamCompleter {
        _scale = scale,
        _framesEmitted = 0,
        _timer = null {
-    codec.then<Null>(_handleCodecReady, onError: (dynamic error, StackTrace stack) {
+    codec.then<void>(_handleCodecReady, onError: (dynamic error, StackTrace stack) {
       FlutterError.reportError(new FlutterErrorDetails(
         exception: error,
         stack: stack,

packages/flutter/lib/src/services/asset_bundle.dart

@@ -181,7 +181,7 @@ abstract class CachingAssetBundle extends AssetBundle {
       return _structuredDataCache[key];
     Completer<T> completer;
     Future<T> result;
-    loadString(key, cache: false).then<T>(parser).then<Null>((T value) {
+    loadString(key, cache: false).then<T>(parser).then<void>((T value) {
       result = new SynchronousFuture<T>(value);
       _structuredDataCache[key] = result;
       if (completer != null) {

packages/flutter/lib/src/widgets/animated_list.dart

@@ -298,7 +298,7 @@ class AnimatedListState extends State<AnimatedList> with TickerProviderStateMixi
       _itemsCount += 1;
     });
 
-    controller.forward().then<Null>((Null value) {
+    controller.forward().then<void>((Null value) {
       _removeActiveItemAt(_incomingItems, incomingItem.itemIndex).controller.dispose();
     });
   }
@@ -333,7 +333,7 @@ class AnimatedListState extends State<AnimatedList> with TickerProviderStateMixi
         ..sort();
     });
 
-    controller.reverse().then<Null>((Null value) {
+    controller.reverse().then<void>((Null value) {
       _removeActiveItemAt(_outgoingItems, outgoingItem.itemIndex).controller.dispose();
 
       // Decrement the incoming and outgoing item indices to account

packages/flutter/lib/src/widgets/async.dart

@@ -513,7 +513,7 @@ class _FutureBuilderState<T> extends State<FutureBuilder<T>> {
     if (widget.future != null) {
       final Object callbackIdentity = new Object();
       _activeCallbackIdentity = callbackIdentity;
-      widget.future.then<Null>((T data) {
+      widget.future.then<void>((T data) {
         if (_activeCallbackIdentity == callbackIdentity) {
           setState(() {
             _snapshot = new AsyncSnapshot<T>.withData(ConnectionState.done, data);

packages/flutter/test/foundation/capture_output.dart

@@ -8,7 +8,7 @@ import 'dart:ui' show VoidCallback;
 List<String> captureOutput(VoidCallback fn) {
   final List<String> log = <String>[];
 
-  runZoned<Null>(fn, zoneSpecification: new ZoneSpecification(
+  runZoned<void>(fn, zoneSpecification: new ZoneSpecification(
     print: (Zone self,
             ZoneDelegate parent,
             Zone zone,

packages/flutter/test/widgets/image_resolution_test.dart

@@ -90,7 +90,7 @@ class TestAssetBundle extends CachingAssetBundle {
 
 class FakeImageStreamCompleter extends ImageStreamCompleter {
   FakeImageStreamCompleter(Future<ImageInfo> image) {
-    image.then<Null>(setImage);
+    image.then<void>(setImage);
   }
 }
 
@@ -100,7 +100,7 @@ class TestAssetImage extends AssetImage {
   @override
   ImageStreamCompleter load(AssetBundleImageKey key) {
     ImageInfo imageInfo;
-    key.bundle.load(key.name).then<Null>((ByteData data) {
+    key.bundle.load(key.name).then<void>((ByteData data) {
       final TestByteData testData = data;
       final ui.Image image = new TestImage(testData.scale);
       imageInfo = new ImageInfo(image: image, scale: key.scale);