Fixed some analysis errors. (#9967)

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

@@ -308,7 +308,7 @@ class RefreshIndicatorState extends State<RefreshIndicator> with TickerProviderS
     _mode = _RefreshIndicatorMode.snap;
     _positionController
       .animateTo(1.0 / _kDragSizeFactorLimit, duration: _kIndicatorSnapDuration)
-      .then((Null value) {
+      .then<Null>((Null value) {
         if (mounted && _mode == _RefreshIndicatorMode.snap) {
           assert(widget.onRefresh != null);
           setState(() {

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

@@ -291,7 +291,7 @@ class AnimatedListState extends State<AnimatedList> with TickerProviderStateMixi
       _itemsCount += 1;
     });
 
-    controller.forward().then((Null value) {
+    controller.forward().then<Null>((Null value) {
       _removeActiveItemAt(_incomingItems, incomingItem.itemIndex).controller.dispose();
     });
   }
@@ -326,7 +326,7 @@ class AnimatedListState extends State<AnimatedList> with TickerProviderStateMixi
         ..sort();
     });
 
-    controller.reverse().then((Null value) {
+    controller.reverse().then<Null>((Null value) {
       _removeActiveItemAt(_outgoingItems, outgoingItem.itemIndex).controller.dispose();
 
       // Decrement the incoming and outgoing item indices to account