More const immutable classes (#9544)

* more widget const constructors

* prefer const constructors

* address review comments

dev/benchmarks/complex_layout/lib/main.dart

@@ -176,9 +176,9 @@ class FancyImageItem extends StatelessWidget {
         new ItemDescription(),
         new ItemImageBox(),
         new InfoBar(),
-        new Padding(
+        const Padding(
           padding: const EdgeInsets.symmetric(horizontal: 8.0),
-          child: new Divider()
+          child: const Divider()
         ),
         new IconBar(),
         new FatDivider()
@@ -198,9 +198,9 @@ class FancyGalleryItem extends StatelessWidget {
         const UserHeader('Ali Connors'),
         new ItemGalleryBox(index),
         new InfoBar(),
-        new Padding(
+        const Padding(
           padding: const EdgeInsets.symmetric(horizontal: 8.0),
-          child: new Divider()
+          child: const Divider()
         ),
         new IconBar(),
         new FatDivider()
@@ -599,7 +599,7 @@ class GalleryDrawer extends StatelessWidget {
               onChanged: (bool value) { _changeTheme(context, value); },
             ),
           ),
-          new Divider(),
+          const Divider(),
           new ListTile(
             leading: const Icon(Icons.hourglass_empty),
             title: const Text('Animate Slowly'),

dev/manual_tests/card_collection.dart

@@ -93,20 +93,20 @@ class CardCollectionState extends State<CardCollection> {
             buildDrawerCheckbox("Fixed size cards", _fixedSizeCards, _toggleFixedSizeCards),
             buildDrawerCheckbox("Let the sun shine", _sunshine, _toggleSunshine),
             buildDrawerCheckbox("Vary font sizes", _varyFontSizes, _toggleVaryFontSizes, enabled: !_editable),
-            new Divider(),
+            const Divider(),
             buildDrawerColorRadioItem("Deep Purple", Colors.deepPurple, _primaryColor, _selectColor),
             buildDrawerColorRadioItem("Green", Colors.green, _primaryColor, _selectColor),
             buildDrawerColorRadioItem("Amber", Colors.amber, _primaryColor, _selectColor),
             buildDrawerColorRadioItem("Teal", Colors.teal, _primaryColor, _selectColor),
-            new Divider(),
+            const Divider(),
             buildDrawerDirectionRadioItem("Dismiss horizontally", DismissDirection.horizontal, _dismissDirection, _changeDismissDirection, icon: Icons.code),
             buildDrawerDirectionRadioItem("Dismiss left", DismissDirection.endToStart, _dismissDirection, _changeDismissDirection, icon: Icons.arrow_back),
             buildDrawerDirectionRadioItem("Dismiss right", DismissDirection.startToEnd, _dismissDirection, _changeDismissDirection, icon: Icons.arrow_forward),
-            new Divider(),
+            const Divider(),
             buildFontRadioItem("Left-align text", TextAlign.left, _textAlign, _changeTextAlign, icon: Icons.format_align_left, enabled: !_editable),
             buildFontRadioItem("Center-align text", TextAlign.center, _textAlign, _changeTextAlign, icon: Icons.format_align_center, enabled: !_editable),
             buildFontRadioItem("Right-align text", TextAlign.right, _textAlign, _changeTextAlign, icon: Icons.format_align_right, enabled: !_editable),
-            new Divider(),
+            const Divider(),
             new ListTile(
               leading: const Icon(Icons.dvr),
               onTap: () { debugDumpApp(); debugDumpRenderTree(); },

examples/flutter_gallery/lib/demo/animation/home.dart

@@ -525,7 +525,7 @@ class _AnimationDemoHomeState extends State<AnimationDemoHome> {
           new Positioned(
             top: statusBarHeight,
             left: 0.0,
-            child: new IconTheme(
+            child: const IconTheme(
               data: const IconThemeData(color: Colors.white),
               child: const BackButton(),
             ),

examples/flutter_gallery/lib/demo/calculator/home.dart

@@ -126,7 +126,7 @@ class _CalculatorState extends State<Calculator> {
             flex: 2,
             child: new CalcDisplay(content: _expression.toString())
           ),
-          new Divider(height: 1.0),
+          const Divider(height: 1.0),
           new Expanded(
             flex: 3,
             child: new KeyPad(calcState: this)

examples/flutter_gallery/lib/demo/contacts_demo.dart

@@ -116,7 +116,7 @@ class ContactsDemoState extends State<ContactsDemo> {
                   icon: const Icon(Icons.create),
                   tooltip: 'Edit',
                   onPressed: () {
-                    _scaffoldKey.currentState.showSnackBar(new SnackBar(
+                    _scaffoldKey.currentState.showSnackBar(const SnackBar(
                       content: const Text('This is actually just a demo. Editing isn\'t supported.')
                     ));
                   },
@@ -176,7 +176,7 @@ class ContactsDemoState extends State<ContactsDemo> {
                       icon: Icons.message,
                       tooltip: 'Send message',
                       onPressed: () {
-                        _scaffoldKey.currentState.showSnackBar(new SnackBar(
+                        _scaffoldKey.currentState.showSnackBar(const SnackBar(
                           content: const Text('Pretend that this opened your SMS application.')
                         ));
                       },
@@ -189,7 +189,7 @@ class ContactsDemoState extends State<ContactsDemo> {
                       icon: Icons.message,
                       tooltip: 'Send message',
                       onPressed: () {
-                        _scaffoldKey.currentState.showSnackBar(new SnackBar(
+                        _scaffoldKey.currentState.showSnackBar(const SnackBar(
                           content: const Text('In this demo, this button doesn\'t do anything.')
                         ));
                       },
@@ -202,7 +202,7 @@ class ContactsDemoState extends State<ContactsDemo> {
                       icon: Icons.message,
                       tooltip: 'Send message',
                       onPressed: () {
-                        _scaffoldKey.currentState.showSnackBar(new SnackBar(
+                        _scaffoldKey.currentState.showSnackBar(const SnackBar(
                           content: const Text('Imagine if you will, a messaging application.')
                         ));
                       },
@@ -220,7 +220,7 @@ class ContactsDemoState extends State<ContactsDemo> {
                       icon: Icons.email,
                       tooltip: 'Send personal e-mail',
                       onPressed: () {
-                        _scaffoldKey.currentState.showSnackBar(new SnackBar(
+                        _scaffoldKey.currentState.showSnackBar(const SnackBar(
                           content: const Text('Here, your e-mail application would open.')
                         ));
                       },
@@ -233,7 +233,7 @@ class ContactsDemoState extends State<ContactsDemo> {
                       icon: Icons.email,
                       tooltip: 'Send work e-mail',
                       onPressed: () {
-                        _scaffoldKey.currentState.showSnackBar(new SnackBar(
+                        _scaffoldKey.currentState.showSnackBar(const SnackBar(
                           content: const Text('This is a demo, so this button does not actually work.')
                         ));
                       },
@@ -251,7 +251,7 @@ class ContactsDemoState extends State<ContactsDemo> {
                       icon: Icons.map,
                       tooltip: 'Open map',
                       onPressed: () {
-                        _scaffoldKey.currentState.showSnackBar(new SnackBar(
+                        _scaffoldKey.currentState.showSnackBar(const SnackBar(
                           content: const Text('This would show a map of San Francisco.')
                         ));
                       },
@@ -265,7 +265,7 @@ class ContactsDemoState extends State<ContactsDemo> {
                       icon: Icons.map,
                       tooltip: 'Open map',
                       onPressed: () {
-                        _scaffoldKey.currentState.showSnackBar(new SnackBar(
+                        _scaffoldKey.currentState.showSnackBar(const SnackBar(
                           content: const Text('This would show a map of Mountain View.')
                         ));
                       },
@@ -279,7 +279,7 @@ class ContactsDemoState extends State<ContactsDemo> {
                       icon: Icons.map,
                       tooltip: 'Open map',
                       onPressed: () {
-                        _scaffoldKey.currentState.showSnackBar(new SnackBar(
+                        _scaffoldKey.currentState.showSnackBar(const SnackBar(
                           content: const Text('This would also show a map, if this was not a demo.')
                         ));
                       },

examples/flutter_gallery/lib/demo/cupertino/cupertino_activity_indicator_demo.dart

@@ -14,8 +14,8 @@ class CupertinoProgressIndicatorDemo extends StatelessWidget {
       appBar: new AppBar(
         title: const Text('Cupertino Activity Indicator'),
       ),
-      body: new Center(
-        child: new CupertinoActivityIndicator(),
+      body: const Center(
+        child: const CupertinoActivityIndicator(),
       ),
     );
   }

examples/flutter_gallery/lib/demo/cupertino/cupertino_buttons_demo.dart

@@ -49,7 +49,7 @@ class _CupertinoButtonDemoState extends State<CupertinoButtonsDemo> {
                           setState(() {_pressedCount++;});
                         }
                       ),
-                      new CupertinoButton(
+                      const CupertinoButton(
                         child: const Text('Disabled'),
                         onPressed: null,
                       ),
@@ -65,7 +65,7 @@ class _CupertinoButtonDemoState extends State<CupertinoButtonsDemo> {
                   }
                 ),
                 const Padding(padding: const EdgeInsets.all(12.0)),
-                new CupertinoButton(
+                const CupertinoButton(
                   child: const Text('Disabled'),
                   color: _kBlue,
                   onPressed: null,

examples/flutter_gallery/lib/demo/material/buttons_demo.dart

@@ -121,7 +121,7 @@ class _ButtonsDemoState extends State<ButtonsDemo> {
               // Perform some action
             },
           ),
-          new FlatButton(
+          const FlatButton(
             child: const Text('DISABLED'),
             onPressed: null,
           )

examples/flutter_gallery/lib/demo/material/drawer_demo.dart

@@ -67,7 +67,7 @@ class _DrawerDemoState extends State<DrawerDemo> with TickerProviderStateMixin {
 
   void _showNotImplementedMessage() {
     Navigator.of(context).pop(); // Dismiss the drawer.
-    _scaffoldKey.currentState.showSnackBar(new SnackBar(
+    _scaffoldKey.currentState.showSnackBar(const SnackBar(
       content: const Text("The drawer's items don't do anything")
     ));
   }

examples/flutter_gallery/lib/demo/material/expansion_panels_demo.dart

@@ -108,7 +108,7 @@ class CollapsibleBody extends StatelessWidget {
             )
           )
         ),
-        new Divider(height: 1.0),
+        const Divider(height: 1.0),
         new Container(
           padding: const EdgeInsets.symmetric(vertical: 16.0),
           child: new Row(

examples/flutter_gallery/lib/demo/material/leave_behind_demo.dart

@@ -103,13 +103,13 @@ class LeaveBehindDemoState extends State<LeaveBehindDemo> {
       },
       background: new Container(
         color: theme.primaryColor,
-        child: new ListTile(
+        child: const ListTile(
           leading: const Icon(Icons.delete, color: Colors.white, size: 36.0)
         )
       ),
       secondaryBackground: new Container(
         color: theme.primaryColor,
-        child: new ListTile(
+        child: const ListTile(
           trailing: const Icon(Icons.archive, color: Colors.white, size: 36.0)
         )
       ),

examples/flutter_gallery/lib/demo/material/menu_demo.dart

@@ -119,31 +119,31 @@ class MenuDemoState extends State<MenuDemo> {
               padding: EdgeInsets.zero,
               onSelected: showMenuSelection,
               itemBuilder: (BuildContext context) => <PopupMenuEntry<String>>[
-                new PopupMenuItem<String>(
+                const PopupMenuItem<String>(
                   value: 'Preview',
-                  child: new ListTile(
+                  child: const ListTile(
                     leading: const Icon(Icons.visibility),
                     title: const Text('Preview')
                   )
                 ),
-                new PopupMenuItem<String>(
+                const PopupMenuItem<String>(
                   value: 'Share',
-                  child: new ListTile(
+                  child: const ListTile(
                     leading: const Icon(Icons.person_add),
                     title: const Text('Share')
                   )
                 ),
-                new PopupMenuItem<String>(
+                const PopupMenuItem<String>(
                   value: 'Get Link',
-                  child: new ListTile(
+                  child: const ListTile(
                     leading: const Icon(Icons.link),
                     title: const Text('Get link')
                   )
                 ),
                 const PopupMenuDivider(), // ignore: list_element_type_not_assignable, https://github.com/flutter/flutter/issues/5771
-                new PopupMenuItem<String>(
+                const PopupMenuItem<String>(
                   value: 'Remove',
-                  child: new ListTile(
+                  child: const ListTile(
                     leading: const Icon(Icons.delete),
                     title: const Text('Remove')
                   )

examples/flutter_gallery/lib/demo/material/slider_demo.dart

@@ -43,7 +43,7 @@ class _SliderDemoState extends State<SliderDemo> {
             new Column(
               mainAxisSize: MainAxisSize.min,
               children: <Widget> [
-                new Slider(value: 0.25, thumbOpenAtMin: true, onChanged: null),
+                const Slider(value: 0.25, thumbOpenAtMin: true, onChanged: null),
                 const Text('Disabled'),
               ]
             ),

examples/flutter_gallery/lib/demo/material/tooltip_demo.dart

@@ -46,7 +46,7 @@ class TooltipDemo extends StatelessWidget {
                   color: theme.iconTheme.color,
                   tooltip: 'Place a phone call',
                   onPressed: () {
-                    Scaffold.of(context).showSnackBar(new SnackBar(
+                    Scaffold.of(context).showSnackBar(const SnackBar(
                        content: const Text('That was an ordinary tap.')
                     ));
                   }

examples/flutter_gallery/lib/demo/material/two_level_list_demo.dart

@@ -14,19 +14,19 @@ class TwoLevelListDemo extends StatelessWidget {
       body: new TwoLevelList(
         type: MaterialListType.oneLine,
         children: <Widget>[
-          new TwoLevelListItem(title: const Text('Top')),
+          const TwoLevelListItem(title: const Text('Top')),
           new TwoLevelSublist(
              title: const Text('Sublist'),
              backgroundColor: Theme.of(context).accentColor.withOpacity(0.025),
              children: <Widget>[
-               new TwoLevelListItem(title: const Text('One')),
-               new TwoLevelListItem(title: const Text('Two')),
+               const TwoLevelListItem(title: const Text('One')),
+               const TwoLevelListItem(title: const Text('Two')),
                // https://en.wikipedia.org/wiki/Free_Four
-               new TwoLevelListItem(title: const Text('Free')),
-               new TwoLevelListItem(title: const Text('Four'))
+               const TwoLevelListItem(title: const Text('Free')),
+               const TwoLevelListItem(title: const Text('Four'))
              ]
           ),
-           new TwoLevelListItem(title: const Text('Bottom'))
+           const TwoLevelListItem(title: const Text('Bottom'))
         ]
       )
     );

examples/flutter_gallery/lib/demo/pesto_demo.dart

@@ -84,7 +84,7 @@ class _RecipeGridPageState extends State<RecipeGridPage> {
         floatingActionButton: new FloatingActionButton(
           child: const Icon(Icons.edit),
           onPressed: () {
-            scaffoldKey.currentState.showSnackBar(new SnackBar(
+            scaffoldKey.currentState.showSnackBar(const SnackBar(
               content: const Text('Not supported.'),
             ));
           },
@@ -108,7 +108,7 @@ class _RecipeGridPageState extends State<RecipeGridPage> {
           icon: const Icon(Icons.search),
           tooltip: 'Search',
           onPressed: () {
-            scaffoldKey.currentState.showSnackBar(new SnackBar(
+            scaffoldKey.currentState.showSnackBar(const SnackBar(
               content: const Text('Not supported.'),
             ));
           },

examples/flutter_gallery/lib/demo/shrine/shrine_page.dart

@@ -82,7 +82,7 @@ class ShrinePageState extends State<ShrinePage> {
 
   void _emptyCart() {
     widget.shoppingCart.clear();
-    widget.scaffoldKey.currentState.showSnackBar(new SnackBar(content: const Text('Shopping cart is empty')));
+    widget.scaffoldKey.currentState.showSnackBar(const SnackBar(content: const Text('Shopping cart is empty')));
   }
 
   @override

examples/flutter_gallery/lib/gallery/drawer.dart

@@ -254,10 +254,10 @@ class GalleryDrawer extends StatelessWidget {
       new GalleryDrawerHeader(light: useLightTheme),
       lightThemeItem,
       darkThemeItem,
-      new Divider(),
+      const Divider(),
       mountainViewItem,
       cupertinoItem,
-      new Divider(),
+      const Divider(),
       animateSlowlyItem,
       // index 8, optional: Performance Overlay
       sendFeedbackItem,

examples/flutter_gallery/lib/gallery/example_code.dart

@@ -44,7 +44,7 @@ new FlatButton(
 // Create a disabled button.
 // Buttons are disabled when onPressed isn't
 // specified or is null.
-new FlatButton(
+const FlatButton(
   child: const Text('BUTTON TITLE'),
   onPressed: null
 );

examples/flutter_gallery/lib/gallery/home.dart

@@ -132,7 +132,7 @@ class GalleryHomeState extends State<GalleryHome> with SingleTickerProviderState
     for (GalleryItem galleryItem in kAllGalleryItems) {
       if (category != galleryItem.category) {
         if (category != null)
-          listItems.add(new Divider());
+          listItems.add(const Divider());
         listItems.add(
           new Container(
             height: 48.0,
@@ -166,7 +166,7 @@ class GalleryHomeState extends State<GalleryHome> with SingleTickerProviderState
       ),
       body: new CustomScrollView(
         slivers: <Widget>[
-          new SliverAppBar(
+          const SliverAppBar(
             pinned: true,
             expandedHeight: _kFlexibleSpaceMaxHeight,
             flexibleSpace: const FlexibleSpaceBar(

examples/stocks/lib/stock_home.dart

@@ -118,12 +118,12 @@ class StockHomeState extends State<StockHome> {
       child: new ListView(
         children: <Widget>[
           const DrawerHeader(child: const Center(child: const Text('Stocks'))),
-          new ListTile(
+          const ListTile(
             leading: const Icon(Icons.assessment),
             title: const Text('Stock List'),
             selected: true,
           ),
-          new ListTile(
+          const ListTile(
             leading: const Icon(Icons.account_balance),
             title: const Text('Account Balance'),
             enabled: false,
@@ -142,7 +142,7 @@ class StockHomeState extends State<StockHome> {
               }
             },
           ),
-          new Divider(),
+          const Divider(),
           new ListTile(
             leading: const Icon(Icons.thumb_up),
             title: const Text('Optimistic'),
@@ -167,7 +167,7 @@ class StockHomeState extends State<StockHome> {
               _handleStockModeChange(StockMode.pessimistic);
             },
           ),
-          new Divider(),
+          const Divider(),
           new ListTile(
             leading: const Icon(Icons.settings),
             title: const Text('Settings'),

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

@@ -13,12 +13,11 @@ import 'package:flutter/widgets.dart';
 ///  * <https://developer.apple.com/ios/human-interface-guidelines/ui-controls/progress-indicators/#activity-indicators>
 class CupertinoActivityIndicator extends StatefulWidget {
   /// Creates an iOS-style activity indicator.
-  CupertinoActivityIndicator({
+  const CupertinoActivityIndicator({
     Key key,
     this.animating: true,
-  }) : super(key: key) {
-    assert(animating != null);
-  }
+  }) : assert(animating != null),
+       super(key: key);
 
   /// Whether the activity indicator is running its animation.
   ///

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

@@ -42,16 +42,14 @@ const EdgeInsets _kBackgroundButtonPadding =
 ///  * <https://developer.apple.com/ios/human-interface-guidelines/ui-controls/buttons/>
 class CupertinoButton extends StatefulWidget {
   /// Creates an iOS-style button.
-  CupertinoButton({
+  const CupertinoButton({
     @required this.child,
     this.padding,
     this.color,
     this.minSize: 44.0,
     this.pressedOpacity: 0.1,
     @required this.onPressed,
-  }) {
-    assert(pressedOpacity >= 0.0 && pressedOpacity <= 1.0);
-  }
+  }) : assert(pressedOpacity >= 0.0 && pressedOpacity <= 1.0);
 
   /// The widget below this widget in the tree.
   ///

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

@@ -177,13 +177,11 @@ const Color _kDestructiveActionColor = const Color(0xFFFF3B30);
 ///  * [CupertinoAlertDialog]
 class CupertinoDialogAction extends StatelessWidget {
   /// Creates an action for an iOS-style dialog.
-  CupertinoDialogAction({
+  const CupertinoDialogAction({
     this.onPressed,
     this.isDestructive: false,
     @required this.child,
-  }) {
-    assert(child != null);
-  }
+  }) : assert(child != null);
 
   /// The callback that is called when the button is tapped or otherwise activated.
   ///

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

@@ -41,7 +41,7 @@ class CupertinoSlider extends StatefulWidget {
   ///
   /// * [value] determines currently selected value for this slider.
   /// * [onChanged] is called when the user selects a new value for the slider.
-  CupertinoSlider({
+  const CupertinoSlider({
     Key key,
     @required this.value,
     @required this.onChanged,
@@ -49,13 +49,12 @@ class CupertinoSlider extends StatefulWidget {
     this.max: 1.0,
     this.divisions,
     this.activeColor: const Color(0xFF027AFF),
-  }) : super(key: key) {
-    assert(value != null);
-    assert(min != null);
-    assert(max != null);
-    assert(value >= min && value <= max);
-    assert(divisions == null || divisions > 0);
-  }
+  }) : assert(value != null),
+       assert(min != null),
+       assert(max != null),
+       assert(value >= min && value <= max),
+       assert(divisions == null || divisions > 0),
+       super(key: key);
 
   /// The currently selected value for this slider.
   ///

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

@@ -677,7 +677,7 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
 ///  * <https://material.google.com/layout/structure.html#structure-toolbars>
 class SliverAppBar extends StatefulWidget {
   /// Creates a material design app bar that can be placed in a [CustomScrollView].
-  SliverAppBar({
+  const SliverAppBar({
     Key key,
     this.leading,
     this.title,
@@ -695,13 +695,12 @@ class SliverAppBar extends StatefulWidget {
     this.floating: false,
     this.pinned: false,
     this.snap: false,
-  }) : super(key: key) {
-    assert(primary != null);
-    assert(floating != null);
-    assert(pinned != null);
-    assert(pinned && floating ? bottom != null : true);
-    assert(snap != null);
-  }
+  }) : assert(primary != null),
+       assert(floating != null),
+       assert(pinned != null),
+       assert(pinned && floating ? bottom != null : true),
+       assert(snap != null),
+       super(key: key);
 
   /// A widget to display before the [title].
   ///

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

@@ -47,15 +47,14 @@ class BottomSheet extends StatefulWidget {
   /// Typically, bottom sheets are created implicitly by
   /// [Scaffold.showBottomSheet], for persistent bottom sheets, or by
   /// [showModalBottomSheet], for modal bottom sheets.
-  BottomSheet({
+  const BottomSheet({
     Key key,
     this.animationController,
     @required this.onClosing,
     @required this.builder
-  }) : super(key: key) {
-    assert(onClosing != null);
-    assert(builder != null);
-  }
+  }) : assert(onClosing != null),
+       assert(builder != null),
+       super(key: key);
 
   /// The animation that controls the bottom sheet's position.
   ///

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

@@ -103,19 +103,18 @@ class _CheckboxState extends State<Checkbox> with TickerProviderStateMixin {
 }
 
 class _CheckboxRenderObjectWidget extends LeafRenderObjectWidget {
-  _CheckboxRenderObjectWidget({
+  const _CheckboxRenderObjectWidget({
     Key key,
     @required this.value,
     @required this.activeColor,
     @required this.inactiveColor,
     @required this.onChanged,
     @required this.vsync,
-  }) : super(key: key) {
-    assert(value != null);
-    assert(activeColor != null);
-    assert(inactiveColor != null);
-    assert(vsync != null);
-  }
+  }) : assert(value != null),
+       assert(activeColor != null),
+       assert(inactiveColor != null),
+       assert(vsync != null),
+       super(key: key);
 
   final bool value;
   final Color activeColor;

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

@@ -44,17 +44,16 @@ const double _kDatePickerLandscapeHeight = _kMaxDayPickerHeight + _kDialogAction
 
 // Shows the selected date in large font and toggles between year and day mode
 class _DatePickerHeader extends StatelessWidget {
-  _DatePickerHeader({
+  const _DatePickerHeader({
     Key key,
     @required this.selectedDate,
     @required this.mode,
     @required this.onModeChanged,
     @required this.orientation,
-  }) : super(key: key) {
-    assert(selectedDate != null);
-    assert(mode != null);
-    assert(orientation != null);
-  }
+  }) : assert(selectedDate != null),
+       assert(mode != null),
+       assert(orientation != null),
+       super(key: key);
 
   final DateTime selectedDate;
   final _DatePickerMode mode;

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

@@ -27,14 +27,13 @@ class Divider extends StatelessWidget {
   /// Creates a material design divider.
   ///
   /// The height must be at least 1.0 logical pixels.
-  Divider({
+  const Divider({
     Key key,
     this.height: 16.0,
     this.indent: 0.0,
     this.color
-  }) : super(key: key) {
-    assert(height >= 1.0);
-  }
+  }) : assert(height >= 1.0),
+       super(key: key);
 
   /// The divider's vertical extent.
   ///

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

@@ -116,12 +116,11 @@ class DrawerController extends StatefulWidget {
   /// Rarely used directly.
   ///
   /// The [child] argument must not be null and is typically a [Drawer].
-  DrawerController({
+  const DrawerController({
     GlobalKey key,
     @required this.child
-  }) : super(key: key) {
-    assert(child != null);
-  }
+  }) : assert(child != null),
+       super(key: key);
 
   /// The widget below this widget in the tree.
   ///

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

@@ -333,13 +333,12 @@ class DropdownMenuItem<T> extends StatelessWidget {
   /// Creates an item for a dropdown menu.
   ///
   /// The [child] argument is required.
-  DropdownMenuItem({
+  const DropdownMenuItem({
     Key key,
     this.value,
     @required this.child,
-  }) : super(key: key) {
-    assert(child != null);
-  }
+  }) : assert(child != null),
+       super(key: key);
 
   /// The widget below this widget in the tree.
   ///
@@ -370,12 +369,11 @@ class DropdownMenuItem<T> extends StatelessWidget {
 class DropdownButtonHideUnderline extends InheritedWidget {
   /// Creates a [DropdownButtonHideUnderline]. A non-null [child] must
   /// be given.
-  DropdownButtonHideUnderline({
+  const DropdownButtonHideUnderline({
     Key key,
     @required Widget child,
-  }) : super(key: key, child: child) {
-    assert(child != null);
-  }
+  }) : assert(child != null),
+       super(key: key, child: child);
 
   /// Returns whether the underline of [DropdownButton] widgets should
   /// be hidden.

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

@@ -21,17 +21,16 @@ import 'theme.dart';
 class ExpandIcon extends StatefulWidget {
   /// Creates an [ExpandIcon] with the given padding, and a callback that is
   /// triggered when the icon is pressed.
-  ExpandIcon({
+  const ExpandIcon({
     Key key,
     this.isExpanded: false,
     this.size: 24.0,
     @required this.onPressed,
     this.padding: const EdgeInsets.all(8.0)
-  }) : super(key: key) {
-    assert(isExpanded != null);
-    assert(size != null);
-    assert(padding != null);
-  }
+  }) : assert(isExpanded != null),
+       assert(size != null),
+       assert(padding != null),
+       super(key: key);
 
   /// Whether the icon is in an expanded state.
   ///

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

@@ -73,15 +73,14 @@ class ExpansionPanel {
 class ExpansionPanelList extends StatelessWidget {
   /// Creates an expansion panel list widget. The [expansionCallback] is
   /// triggered when an expansion panel expand/collapse button is pushed.
-  ExpansionPanelList({
+  const ExpansionPanelList({
     Key key,
     this.children: const <ExpansionPanel>[],
     this.expansionCallback,
     this.animationDuration: kThemeAnimationDuration
-  }) : super(key: key) {
-    assert(children != null);
-    assert(animationDuration != null);
-  }
+  }) : assert(children != null),
+       assert(animationDuration != null),
+       super(key: key);
 
   /// The children of the expansion panel list. They are layed in a similar
   /// fashion to [ListBody].

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

@@ -47,7 +47,7 @@ class FlatButton extends StatelessWidget {
   ///
   /// The [child] argument is required and is typically a [Text] widget in all
   /// caps.
-  FlatButton({
+  const FlatButton({
     Key key,
     @required this.onPressed,
     this.textColor,
@@ -59,9 +59,8 @@ class FlatButton extends StatelessWidget {
     this.textTheme,
     this.colorBrightness,
     @required this.child
-  }) : super(key: key) {
-    assert(child != null);
-  }
+  }) : assert(child != null),
+       super(key: key);
 
   /// The callback that is called when the button is tapped or otherwise activated.
   ///

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

@@ -21,14 +21,13 @@ class GridTile extends StatelessWidget {
   /// Creates a grid tile.
   ///
   /// Must have a child. Does not typically have both a header and a footer.
-  GridTile({
+  const GridTile({
     Key key,
     this.header,
     this.footer,
     @required this.child,
-  }) : super(key: key) {
-    assert(child != null);
-  }
+  }) : assert(child != null),
+       super(key: key);
 
   /// The widget to show over the top of this grid tile.
   ///

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

@@ -16,14 +16,13 @@ class IconTheme extends InheritedWidget {
   /// descendant widgets.
   ///
   /// Both [data] and [child] arguments must not be null.
-  IconTheme({
+  const IconTheme({
     Key key,
     @required this.data,
     @required Widget child
-  }) : super(key: key, child: child) {
-    assert(data != null);
-    assert(child != null);
-  }
+  }) : assert(data != null),
+       assert(child != null),
+       super(key: key, child: child);
 
   /// Creates an icon theme that controls the color, opacity, and size of
   /// descendant widgets, and merges in the current icon theme, if any.

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

@@ -463,15 +463,14 @@ class InputDecorator extends StatelessWidget {
 // Smoothly animate the label of an InputDecorator as the label
 // transitions between inline and caption.
 class _AnimatedLabel extends ImplicitlyAnimatedWidget {
-  _AnimatedLabel({
+  const _AnimatedLabel({
     Key key,
     this.text,
     @required this.style,
     Curve curve: Curves.linear,
     @required Duration duration,
-  }) : super(key: key, curve: curve, duration: duration) {
-    assert(style != null);
-  }
+  }) : assert(style != null),
+       super(key: key, curve: curve, duration: duration);
 
   final String text;
   final TextStyle style;

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

@@ -158,7 +158,7 @@ class ListTile extends StatelessWidget {
   /// If [isThreeLine] is true, then [subtitle] must not be null.
   ///
   /// Requires one of its ancestors to be a [Material] widget.
-  ListTile({
+  const ListTile({
     Key key,
     this.leading,
     this.title,
@@ -170,11 +170,10 @@ class ListTile extends StatelessWidget {
     this.onTap,
     this.onLongPress,
     this.selected: false,
-  }) : super(key: key) {
-    assert(isThreeLine != null);
-    assert(enabled != null);
-    assert(selected != null);
-  }
+  }) : assert(isThreeLine != null),
+       assert(enabled != null),
+       assert(selected != null),
+       super(key: key);
 
   /// A widget to display before the title.
   ///

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

@@ -110,7 +110,7 @@ class Material extends StatefulWidget {
   /// Creates a piece of material.
   ///
   /// The [type] and the [elevation] arguments must not be null.
-  Material({
+  const Material({
     Key key,
     this.type: MaterialType.canvas,
     this.elevation: 0,
@@ -118,11 +118,10 @@ class Material extends StatefulWidget {
     this.textStyle,
     this.borderRadius,
     this.child,
-  }) : super(key: key) {
-    assert(type != null);
-    assert(elevation != null);
-    assert(type != MaterialType.circle || borderRadius == null);
-  }
+  }) : assert(type != null),
+       assert(elevation != null),
+       assert(!(identical(type, MaterialType.circle) && borderRadius != null)),
+       super(key: key);
 
   /// The widget below this widget in the tree.
   final Widget child;

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

@@ -34,12 +34,11 @@ abstract class MergeableMaterialItem {
 class MaterialSlice extends MergeableMaterialItem {
   /// Creates a slice of [Material] that's mergeable within a
   /// [MergeableMaterial].
-  MaterialSlice({
+  const MaterialSlice({
     @required LocalKey key,
     @required this.child,
-  }) : super(key) {
-    assert(key != null);
-  }
+  }) : assert(key != null),
+       super(key);
 
   /// The contents of this slice.
   final Widget child;
@@ -55,12 +54,11 @@ class MaterialSlice extends MergeableMaterialItem {
 /// All [MaterialGap] objects need a [LocalKey].
 class MaterialGap extends MergeableMaterialItem {
   /// Creates a Material gap with a given size.
-  MaterialGap({
+  const MaterialGap({
     @required LocalKey key,
     this.size: 16.0
-  }) : super(key) {
-    assert(key != null);
-  }
+  }) : assert(key != null),
+       super(key);
 
   /// The main axis extent of this gap. For example, if the [MergableMaterial]
   /// is vertical, then this is the height of the gap.

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

@@ -475,7 +475,7 @@ class PopupMenuButton<T> extends StatefulWidget {
   /// Creates a button that shows a popup menu.
   ///
   /// The [itemBuilder] argument must not be null.
-  PopupMenuButton({
+  const PopupMenuButton({
     Key key,
     @required this.itemBuilder,
     this.initialValue,
@@ -484,9 +484,8 @@ class PopupMenuButton<T> extends StatefulWidget {
     this.elevation: 8,
     this.padding: const EdgeInsets.all(8.0),
     this.child
-  }) : super(key: key) {
-    assert(itemBuilder != null);
-  }
+  }) : assert(itemBuilder != null),
+       super(key: key);
 
   /// Called when the button is pressed to create the items to show in the menu.
   final PopupMenuItemBuilder<T> itemBuilder;

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

@@ -127,19 +127,18 @@ class _RadioState<T> extends State<Radio<T>> with TickerProviderStateMixin {
 }
 
 class _RadioRenderObjectWidget extends LeafRenderObjectWidget {
-  _RadioRenderObjectWidget({
+  const _RadioRenderObjectWidget({
     Key key,
     @required this.selected,
     @required this.activeColor,
     @required this.inactiveColor,
     this.onChanged,
     @required this.vsync,
-  }) : super(key: key) {
-    assert(selected != null);
-    assert(activeColor != null);
-    assert(inactiveColor != null);
-    assert(vsync != null);
-  }
+  }) : assert(selected != null),
+       assert(activeColor != null),
+       assert(inactiveColor != null),
+       assert(vsync != null),
+       super(key: key);
 
   final bool selected;
   final Color inactiveColor;

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

@@ -80,17 +80,16 @@ class RefreshIndicator extends StatefulWidget {
   ///
   /// The [onRefresh] and [child] arguments must be non-null. The default
   /// [displacement] is 40.0 logical pixels.
-  RefreshIndicator({
+  const RefreshIndicator({
     Key key,
     @required this.child,
     this.displacement: 40.0,
     @required this.onRefresh,
     this.color,
     this.backgroundColor
-  }) : super(key: key) {
-    assert(child != null);
-    assert(onRefresh != null);
-  }
+  }) : assert(child != null),
+       assert(onRefresh != null),
+       super(key: key);
 
   /// The refresh indicator will be stacked on top of this child. The indicator
   /// will appear when child's Scrollable descendant is over-scrolled.

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

@@ -1015,12 +1015,11 @@ class PersistentBottomSheetController<T> extends ScaffoldFeatureController<_Pers
 }
 
 class _ScaffoldScope extends InheritedWidget {
-  _ScaffoldScope({
+  const _ScaffoldScope({
     @required this.hasDrawer,
     @required Widget child,
-  }) : super(child: child) {
-    assert(hasDrawer != null);
-  }
+  }) : assert(hasDrawer != null),
+       super(child: child);
 
   final bool hasDrawer;
 

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

@@ -49,7 +49,7 @@ class Slider extends StatefulWidget {
   ///
   /// * [value] determines currently selected value for this slider.
   /// * [onChanged] is called when the user selects a new value for the slider.
-  Slider({
+  const Slider({
     Key key,
     @required this.value,
     @required this.onChanged,
@@ -59,14 +59,13 @@ class Slider extends StatefulWidget {
     this.label,
     this.activeColor,
     this.thumbOpenAtMin: false,
-  }) : super(key: key) {
-    assert(value != null);
-    assert(min != null);
-    assert(max != null);
-    assert(value >= min && value <= max);
-    assert(divisions == null || divisions > 0);
-    assert(thumbOpenAtMin != null);
-  }
+  }) : assert(value != null),
+       assert(min != null),
+       assert(max != null),
+       assert(value >= min && value <= max),
+       assert(divisions == null || divisions > 0),
+       assert(thumbOpenAtMin != null),
+       super(key: key);
 
   /// The currently selected value for this slider.
   ///

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

@@ -79,14 +79,13 @@ class SnackBarAction extends StatefulWidget {
   /// Creates an action for a [SnackBar].
   ///
   /// The [label] and [onPressed] arguments must be non-null.
-  SnackBarAction({
+  const SnackBarAction({
     Key key,
     @required this.label,
     @required this.onPressed
-  }) : super(key: key) {
-    assert(label != null);
-    assert(onPressed != null);
-  }
+  }) : assert(label != null),
+       assert(onPressed != null),
+       super(key: key);
 
   /// The button label.
   final String label;
@@ -145,16 +144,15 @@ class SnackBar extends StatelessWidget {
   /// Creates a snack bar.
   ///
   /// The [content] argument must be non-null.
-  SnackBar({
+  const SnackBar({
     Key key,
     @required this.content,
     this.backgroundColor,
     this.action,
     this.duration: _kSnackBarDisplayDuration,
     this.animation,
-  }) : super(key: key) {
-    assert(content != null);
-  }
+  }) : assert(content != null),
+       super(key: key);
 
   /// The primary content of the snack bar.
   ///

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

@@ -80,17 +80,15 @@ class Step {
   /// Creates a step for a [Stepper].
   ///
   /// The [title], [content], and [state] arguments must not be null.
-  Step({
+  const Step({
     @required this.title,
     this.subtitle,
     @required this.content,
     this.state: StepState.indexed,
     this.isActive: false,
-  }) {
-    assert(title != null);
-    assert(content != null);
-    assert(state != null);
-  }
+  }) : assert(title != null),
+       assert(content != null),
+       assert(state != null);
 
   /// The title of the step that typically describes it.
   final Widget title;

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

@@ -39,13 +39,12 @@ const EdgeInsets _kTabLabelPadding = const EdgeInsets.symmetric(horizontal: 12.0
 class Tab extends StatelessWidget {
   /// Creates a material design [TabBar] tab. At least one of [text] and [icon]
   /// must be non-null.
-  Tab({
+  const Tab({
     Key key,
     this.text,
     this.icon,
-  }) : super(key: key) {
-    assert(text != null || icon != null);
-  }
+  }) : assert(text != null || icon != null),
+       super(key: key);
 
   /// The text to display as the tab's label.
   final String text;

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

@@ -31,15 +31,14 @@ class Theme extends InheritedWidget {
   /// Applies the given theme [data] to [child].
   ///
   /// The [data] and [child] arguments must not be null.
-  Theme({
+  const Theme({
     Key key,
     @required this.data,
     this.isMaterialAppTheme: false,
     @required Widget child
-  }) : super(key: key, child: child) {
-    assert(child != null);
-    assert(data != null);
-  }
+  }) : assert(child != null),
+       assert(data != null),
+       super(key: key, child: child);
 
   /// Specifies the color and typography values for descendant widgets.
   final ThemeData data;
@@ -154,17 +153,16 @@ class AnimatedTheme extends ImplicitlyAnimatedWidget {
   ///
   /// By default, the theme transition uses a linear curve. The [data] and
   /// [child] arguments must not be null.
-  AnimatedTheme({
+  const AnimatedTheme({
     Key key,
     @required this.data,
     this.isMaterialAppTheme: false,
     Curve curve: Curves.linear,
     Duration duration: kThemeAnimationDuration,
     @required this.child,
-  }) : super(key: key, curve: curve, duration: duration) {
-    assert(child != null);
-    assert(data != null);
-  }
+  }) : assert(child != null),
+       assert(data != null),
+       super(key: key, curve: curve, duration: duration);
 
   /// Specifies the color and typography values for descendant widgets.
   final ThemeData data;

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

@@ -184,7 +184,7 @@ class ThemeData {
   /// This will rarely be used directly. It is used by [lerp] to
   /// create intermediate themes based on two themes created with the
   /// [new ThemeData] constructor.
-  ThemeData.raw({
+  const ThemeData.raw({
     @required this.brightness,
     @required this.primaryColor,
     @required this.primaryColorBrightness,
@@ -215,38 +215,36 @@ class ThemeData {
     @required this.primaryIconTheme,
     @required this.accentIconTheme,
     @required this.platform
-  }) {
-    assert(brightness != null);
-    assert(primaryColor != null);
-    assert(primaryColorBrightness != null);
-    assert(accentColor != null);
-    assert(accentColorBrightness != null);
-    assert(canvasColor != null);
-    assert(scaffoldBackgroundColor != null);
-    assert(cardColor != null);
-    assert(dividerColor != null);
-    assert(highlightColor != null);
-    assert(splashColor != null);
-    assert(selectedRowColor != null);
-    assert(unselectedWidgetColor != null);
-    assert(disabledColor != null);
-    assert(buttonColor != null);
-    assert(secondaryHeaderColor != null);
-    assert(textSelectionColor != null);
-    assert(textSelectionHandleColor != null);
-    assert(backgroundColor != null);
-    assert(dialogBackgroundColor != null);
-    assert(indicatorColor != null);
-    assert(hintColor != null);
-    assert(errorColor != null);
-    assert(textTheme != null);
-    assert(primaryTextTheme != null);
-    assert(accentTextTheme != null);
-    assert(iconTheme != null);
-    assert(primaryIconTheme != null);
-    assert(accentIconTheme != null);
-    assert(platform != null);
-  }
+  }) : assert(brightness != null),
+       assert(primaryColor != null),
+       assert(primaryColorBrightness != null),
+       assert(accentColor != null),
+       assert(accentColorBrightness != null),
+       assert(canvasColor != null),
+       assert(scaffoldBackgroundColor != null),
+       assert(cardColor != null),
+       assert(dividerColor != null),
+       assert(highlightColor != null),
+       assert(splashColor != null),
+       assert(selectedRowColor != null),
+       assert(unselectedWidgetColor != null),
+       assert(disabledColor != null),
+       assert(buttonColor != null),
+       assert(secondaryHeaderColor != null),
+       assert(textSelectionColor != null),
+       assert(textSelectionHandleColor != null),
+       assert(backgroundColor != null),
+       assert(dialogBackgroundColor != null),
+       assert(indicatorColor != null),
+       assert(hintColor != null),
+       assert(errorColor != null),
+       assert(textTheme != null),
+       assert(primaryTextTheme != null),
+       assert(accentTextTheme != null),
+       assert(iconTheme != null),
+       assert(primaryIconTheme != null),
+       assert(accentIconTheme != null),
+       assert(platform != null);
 
   /// A default light blue theme.
   factory ThemeData.light() => new ThemeData(brightness: Brightness.light);

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

@@ -207,17 +207,15 @@ class _TimePickerHeaderLayout extends MultiChildLayoutDelegate {
 
 // TODO(ianh): Localize!
 class _TimePickerHeader extends StatelessWidget {
-  _TimePickerHeader({
+  const _TimePickerHeader({
     @required this.selectedTime,
     @required this.mode,
     @required this.orientation,
     @required this.onModeChanged,
     @required this.onChanged,
-  }) {
-    assert(selectedTime != null);
-    assert(mode != null);
-    assert(orientation != null);
-  }
+  }) : assert(selectedTime != null),
+       assert(mode != null),
+       assert(orientation != null);
 
   final TimeOfDay selectedTime;
   final _TimePickerMode mode;
@@ -447,13 +445,11 @@ class _DialPainter extends CustomPainter {
 }
 
 class _Dial extends StatefulWidget {
-  _Dial({
+  const _Dial({
     @required this.selectedTime,
     @required this.mode,
     @required this.onChanged
-  }) {
-    assert(selectedTime != null);
-  }
+  }) : assert(selectedTime != null);
 
   final TimeOfDay selectedTime;
   final _TimePickerMode mode;
@@ -625,12 +621,11 @@ class _DialState extends State<_Dial> with SingleTickerProviderStateMixin {
 }
 
 class _TimePickerDialog extends StatefulWidget {
-  _TimePickerDialog({
+  const _TimePickerDialog({
     Key key,
     @required this.initialTime
-  }) : super(key: key) {
-    assert(initialTime != null);
-  }
+  }) : assert(initialTime != null),
+       super(key: key);
 
   final TimeOfDay initialTime;
 

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

@@ -41,7 +41,7 @@ class Tooltip extends StatefulWidget {
   /// user long presses on the widget.
   ///
   /// The [message] argument cannot be null.
-  Tooltip({
+  const Tooltip({
     Key key,
     @required this.message,
     this.height: 32.0,
@@ -49,14 +49,13 @@ class Tooltip extends StatefulWidget {
     this.verticalOffset: 24.0,
     this.preferBelow: true,
     @required this.child,
-  }) : super(key: key) {
-    assert(message != null);
-    assert(height != null);
-    assert(padding != null);
-    assert(verticalOffset != null);
-    assert(preferBelow != null);
-    assert(child != null);
-  }
+  }) : assert(message != null),
+       assert(height != null),
+       assert(padding != null),
+       assert(verticalOffset != null),
+       assert(preferBelow != null),
+       assert(child != null),
+       super(key: key);
 
   /// The text to display in the tooltip.
   final String message;

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

@@ -29,7 +29,7 @@ const Duration _kExpand = const Duration(milliseconds: 200);
 ///  * [ListTile]
 class TwoLevelListItem extends StatelessWidget {
   /// Creates an item in a two-level list.
-  TwoLevelListItem({
+  const TwoLevelListItem({
     Key key,
     this.leading,
     @required this.title,
@@ -37,9 +37,8 @@ class TwoLevelListItem extends StatelessWidget {
     this.enabled: true,
     this.onTap,
     this.onLongPress
-  }) : super(key: key) {
-    assert(title != null);
-  }
+  }) : assert(title != null),
+       super(key: key);
 
   /// A widget to display before the title.
   ///
@@ -259,14 +258,13 @@ class TwoLevelList extends StatelessWidget {
   /// Creates a scrollable list of items that can expand and collapse.
   ///
   /// The [type] argument must not be null.
-  TwoLevelList({
+  const TwoLevelList({
     Key key,
     this.children: const <Widget>[],
     this.type: MaterialListType.twoLine,
     this.padding
-  }) : super(key: key) {
-    assert(type != null);
-  }
+  }) : assert(type != null),
+       super(key: key);
 
   /// The widgets to display in this list.
   ///

packages/flutter/lib/src/rendering/editable.dart

@@ -32,9 +32,8 @@ class TextSelectionPoint {
   /// Creates a description of a point in a text selection.
   ///
   /// The [point] argument must not be null.
-  TextSelectionPoint(this.point, this.direction) {
-    assert(point != null);
-  }
+  const TextSelectionPoint(this.point, this.direction)
+      : assert(point != null);
 
   /// Screen coordinates of the lower left or lower right corner of the selection.
   final Offset point;

packages/flutter/lib/src/rendering/semantics.dart

@@ -60,18 +60,16 @@ class SemanticsData {
   /// Creates a semantics data object.
   ///
   /// The [flags], [actions], [label], and [Rect] arguments must not be null.
-  SemanticsData({
+  const SemanticsData({
     @required this.flags,
     @required this.actions,
     @required this.label,
     @required this.rect,
     this.transform
-  }) {
-    assert(flags != null);
-    assert(actions != null);
-    assert(label != null);
-    assert(rect != null);
-  }
+  }) : assert(flags != null),
+       assert(actions != null),
+       assert(label != null),
+       assert(rect != null);
 
   /// A bit field of [SemanticsFlags] that apply to this node.
   final int flags;

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

@@ -44,7 +44,7 @@ class AnimatedCrossFade extends StatefulWidget {
   /// The [duration] of the animation is the same for all components (fade in,
   /// fade out, and size), and you can pass [Interval]s instead of [Curve]s in
   /// order to have finer control, e.g., creating an overlap between the fades.
-  AnimatedCrossFade({
+  const AnimatedCrossFade({
     Key key,
     @required this.firstChild,
     @required this.secondChild,
@@ -53,11 +53,10 @@ class AnimatedCrossFade extends StatefulWidget {
     this.sizeCurve: Curves.linear,
     @required this.crossFadeState,
     @required this.duration
-  }) : super(key: key) {
-    assert(firstCurve != null);
-    assert(secondCurve != null);
-    assert(sizeCurve != null);
-  }
+  }) : assert(firstCurve != null),
+       assert(secondCurve != null),
+       assert(sizeCurve != null),
+       super(key: key);
 
   /// The child that is visible when [crossFadeState] is [showFirst]. It fades
   /// out when transitioning from [showFirst] to [showSecond] and vice versa.

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

@@ -38,7 +38,7 @@ typedef Future<LocaleQueryData> LocaleChangedCallback(Locale locale);
 class WidgetsApp extends StatefulWidget {
   /// Creates a widget that wraps a number of widgets that are commonly
   /// required for an application.
-  WidgetsApp({
+  const WidgetsApp({
     Key key,
     @required this.onGenerateRoute,
     this.title,
@@ -51,13 +51,12 @@ class WidgetsApp extends StatefulWidget {
     this.checkerboardRasterCacheImages: false,
     this.showSemanticsDebugger: false,
     this.debugShowCheckedModeBanner: true
-  }) : super(key: key) {
-    assert(color != null);
-    assert(onGenerateRoute != null);
-    assert(showPerformanceOverlay != null);
-    assert(checkerboardRasterCacheImages != null);
-    assert(showSemanticsDebugger != null);
-  }
+  }) : assert(color != null),
+       assert(onGenerateRoute != null),
+       assert(showPerformanceOverlay != null),
+       assert(checkerboardRasterCacheImages != null),
+       assert(showSemanticsDebugger != null),
+       super(key: key);
 
   /// A one-line description of this app for use in the window manager.
   final String title;

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

@@ -187,19 +187,18 @@ enum ConnectionState {
 class AsyncSnapshot<T> {
   /// Creates an [AsyncSnapshot] with the specified [connectionState],
   /// and optionally either [data] or [error] (but not both).
-  AsyncSnapshot._(this.connectionState, this.data, this.error) {
-    assert(connectionState != null);
-    assert(data == null || error == null);
-  }
+  const AsyncSnapshot._(this.connectionState, this.data, this.error)
+      : assert(connectionState != null),
+        assert(!(data != null && error != null));
 
   /// Creates an [AsyncSnapshot] in [ConnectionState.none] with null data and error.
-  AsyncSnapshot.nothing() : this._(ConnectionState.none, null, null);
+  const AsyncSnapshot.nothing() : this._(ConnectionState.none, null, null);
 
   /// Creates an [AsyncSnapshot] in the specified [state] and with the specified [data].
-  AsyncSnapshot.withData(ConnectionState state, T data) : this._(state, data, null);
+  AsyncSnapshot.withData(ConnectionState state, T data) : this._(state, data, null); // not const because https://github.com/dart-lang/sdk/issues/29432
 
   /// Creates an [AsyncSnapshot] in the specified [state] and with the specified [error].
-  AsyncSnapshot.withError(ConnectionState state, Object error) : this._(state, null, error);
+  const AsyncSnapshot.withError(ConnectionState state, Object error) : this._(state, null, error);
 
   /// Current state of connection to the asynchronous computation.
   final ConnectionState connectionState;
@@ -309,19 +308,18 @@ class StreamBuilder<T> extends StreamBuilderBase<T, AsyncSnapshot<T>> {
   /// Creates a new [StreamBuilder] that builds itself based on the latest
   /// snapshot of interaction with the specified [stream] and whose build
   /// strategy is given by [builder].
-  StreamBuilder({
+  const StreamBuilder({
     Key key,
     Stream<T> stream,
     @required this.builder
-  }) : super(key: key, stream: stream) {
-    assert(builder != null);
-  }
+  }) : assert(builder != null),
+       super(key: key, stream: stream);
 
   /// The build strategy currently used by this builder. Cannot be null.
   final AsyncWidgetBuilder<T> builder;
 
   @override
-  AsyncSnapshot<T> initial() => new AsyncSnapshot<T>.nothing();
+  AsyncSnapshot<T> initial() => new AsyncSnapshot<T>.nothing(); // ignore: prefer_const_constructors
 
   @override
   AsyncSnapshot<T> afterConnected(AsyncSnapshot<T> current) => current.inState(ConnectionState.waiting);
@@ -387,13 +385,12 @@ class FutureBuilder<T> extends StatefulWidget {
   /// interaction with a [Future].
   ///
   /// The [builder] must not be null.
-  FutureBuilder({
+  const FutureBuilder({
     Key key,
     this.future,
     @required this.builder
-  }) : super(key: key) {
-    assert(builder != null);
-  }
+  }) : assert(builder != null),
+       super(key: key);
 
   /// The asynchronous computation to which this builder is currently connected,
   /// possibly null.
@@ -412,7 +409,7 @@ class _FutureBuilderState<T> extends State<FutureBuilder<T>> {
   /// calling setState from stale callbacks, e.g. after disposal of this state,
   /// or after widget reconfiguration to a new Future.
   Object _activeCallbackIdentity;
-  AsyncSnapshot<T> _snapshot = new AsyncSnapshot<T>.nothing();
+  AsyncSnapshot<T> _snapshot = new AsyncSnapshot<T>.nothing(); // ignore: prefer_const_constructors
 
   @override
   void initState() {

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

@@ -71,7 +71,7 @@ class Dismissible extends StatefulWidget {
   /// which means the item after the dismissed item would be synced with the
   /// state of the dismissed item. Using keys causes the widgets to sync
   /// according to their keys and avoids this pitfall.
-  Dismissible({
+  const Dismissible({
     @required Key key,
     @required this.child,
     this.background,
@@ -81,10 +81,9 @@ class Dismissible extends StatefulWidget {
     this.direction: DismissDirection.horizontal,
     this.resizeDuration: const Duration(milliseconds: 300),
     this.dismissThresholds: const <DismissDirection, double>{},
-  }) : super(key: key) {
-    assert(key != null);
-    assert(secondaryBackground != null ? background != null : true);
-  }
+  }) : assert(key != null),
+       assert(secondaryBackground != null ? background != null : true),
+       super(key: key);
 
   /// The widget below this widget in the tree.
   final Widget child;

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

@@ -117,7 +117,7 @@ class EditableText extends StatefulWidget {
   ///
   /// The [controller], [focusNode], [style], and [cursorColor] arguments must
   /// not be null.
-  EditableText({
+  const EditableText({
     Key key,
     @required this.controller,
     @required this.focusNode,
@@ -133,15 +133,14 @@ class EditableText extends StatefulWidget {
     this.keyboardType,
     this.onChanged,
     this.onSubmitted,
-  }) : super(key: key) {
-    assert(controller != null);
-    assert(focusNode != null);
-    assert(obscureText != null);
-    assert(style != null);
-    assert(cursorColor != null);
-    assert(maxLines != null);
-    assert(autofocus != null);
-  }
+  }) : assert(controller != null),
+       assert(focusNode != null),
+       assert(obscureText != null),
+       assert(style != null),
+       assert(cursorColor != null),
+       assert(maxLines != null),
+       assert(autofocus != null),
+       super(key: key);
 
   /// Controls the text being edited.
   final TextEditingController controller;

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

@@ -10,13 +10,12 @@ import 'focus_manager.dart';
 import 'framework.dart';
 
 class _FocusScopeMarker extends InheritedWidget {
-  _FocusScopeMarker({
+  const _FocusScopeMarker({
     Key key,
     @required this.node,
     Widget child,
-  }) : super(key: key, child: child) {
-    assert(node != null);
-  }
+  }) : assert(node != null),
+       super(key: key, child: child);
 
   final FocusScopeNode node;
 
@@ -53,15 +52,14 @@ class FocusScope extends StatefulWidget {
   /// Creates a scope in which widgets can receive focus.
   ///
   /// The [node] argument must not be null.
-  FocusScope({
+  const FocusScope({
     Key key,
     @required this.node,
     this.autofocus: false,
     this.child,
-  }) : super(key: key) {
-    assert(node != null);
-    assert(autofocus != null);
-  }
+  }) : assert(node != null),
+       assert(autofocus != null),
+       super(key: key);
 
   /// Controls whether this scope is currently active.
   final FocusScopeNode node;

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

@@ -21,14 +21,13 @@ class Form extends StatefulWidget {
   /// Creates a container for form fields.
   ///
   /// The [child] argument must not be null.
-  Form({
+  const Form({
     Key key,
     @required this.child,
     this.autovalidate: false,
     this.onWillPop,
-  }) : super(key: key) {
-    assert(child != null);
-  }
+  }) : assert(child != null),
+       super(key: key);
 
   /// Returns the closest [FormState] which encloses the given context.
   ///
@@ -194,16 +193,15 @@ class FormField<T> extends StatefulWidget {
   /// Creates a single form field.
   ///
   /// The [builder] argument must not be null.
-  FormField({
+  const FormField({
     Key key,
     @required this.builder,
     this.onSaved,
     this.validator,
     this.initialValue,
     this.autovalidate: false,
-  }) : super(key: key) {
-    assert(builder != null);
-  }
+  }) : assert(builder != null),
+       super(key: key);
 
   /// An optional method to call with the final value when the form is saved via
   /// Form.save().

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

@@ -325,16 +325,15 @@ class RawGestureDetector extends StatefulWidget {
   ///
   /// By default, gesture detectors contribute semantic information to the tree
   /// that is used by assistive technology.
-  RawGestureDetector({
+  const RawGestureDetector({
     Key key,
     this.child,
     this.gestures: const <Type, GestureRecognizerFactory>{},
     this.behavior,
     this.excludeFromSemantics: false
-  }) : super(key: key) {
-    assert(gestures != null);
-    assert(excludeFromSemantics != null);
-  }
+  }) : assert(gestures != null),
+       assert(excludeFromSemantics != null),
+       super(key: key);
 
   /// The widget below this widget in the tree.
   final Widget child;

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

@@ -77,14 +77,13 @@ class Hero extends StatefulWidget {
   /// Create a hero.
   ///
   /// The [tag] and [child] parameters must not be null.
-  Hero({
+  const Hero({
     Key key,
     @required this.tag,
     @required this.child,
-  }) : super(key: key) {
-    assert(tag != null);
-    assert(child != null);
-  }
+  }) : assert(tag != null),
+       assert(child != null),
+       super(key: key);
 
   /// The identifier for this particular hero. If the tag of this hero matches
   /// the tag of a hero on a [PageRoute] that we're navigating to or from, then

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

@@ -63,7 +63,7 @@ class Image extends StatefulWidget {
   /// [new Image.network] and [new Image.asset] respectively.
   ///
   /// The [image] and [repeat] arguments must not be null.
-  Image({
+  const Image({
     Key key,
     @required this.image,
     this.width,
@@ -75,9 +75,8 @@ class Image extends StatefulWidget {
     this.repeat: ImageRepeat.noRepeat,
     this.centerSlice,
     this.gaplessPlayback: false
-  }) : super(key: key) {
-    assert(image != null);
-  }
+  }) : assert(image != null),
+       super(key: key);
 
   /// Creates a widget that displays an [ImageStream] obtained from the network.
   ///

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

@@ -88,14 +88,13 @@ abstract class ImplicitlyAnimatedWidget extends StatefulWidget {
   /// Initializes fields for subclasses.
   ///
   /// The [curve] and [duration] arguments must not be null.
-  ImplicitlyAnimatedWidget({
+  const ImplicitlyAnimatedWidget({
     Key key,
     this.curve: Curves.linear,
     @required this.duration
-  }) : super(key: key) {
-    assert(curve != null);
-    assert(duration != null);
-  }
+  }) : assert(curve != null),
+       assert(duration != null),
+       super(key: key);
 
   /// The curve to apply when animating the parameters of this container.
   final Curve curve;
@@ -386,7 +385,7 @@ class AnimatedPositioned extends ImplicitlyAnimatedWidget {
   /// the three must be null.
   ///
   /// The [curve] and [duration] arguments must not be null.
-  AnimatedPositioned({
+  const AnimatedPositioned({
     Key key,
     @required this.child,
     this.left,
@@ -397,10 +396,9 @@ class AnimatedPositioned extends ImplicitlyAnimatedWidget {
     this.height,
     Curve curve: Curves.linear,
     @required Duration duration,
-  }) : super(key: key, curve: curve, duration: duration) {
-    assert(left == null || right == null || width == null);
-    assert(top == null || bottom == null || height == null);
-  }
+  }) : assert(left == null || right == null || width == null),
+       assert(top == null || bottom == null || height == null),
+      super(key: key, curve: curve, duration: duration);
 
   /// Creates a widget that animates the rectangle it occupies implicitly.
   ///
@@ -526,15 +524,14 @@ class AnimatedOpacity extends ImplicitlyAnimatedWidget {
   ///
   /// The [opacity] argument must not be null and must be between 0.0 and 1.0,
   /// inclusive. The [curve] and [duration] arguments must not be null.
-  AnimatedOpacity({
+  const AnimatedOpacity({
     Key key,
     this.child,
     @required this.opacity,
     Curve curve: Curves.linear,
     @required Duration duration,
-  }) : super(key: key, curve: curve, duration: duration) {
-    assert(opacity != null && opacity >= 0.0 && opacity <= 1.0);
-  }
+  }) : assert(opacity != null && opacity >= 0.0 && opacity <= 1.0),
+       super(key: key, curve: curve, duration: duration);
 
   /// The widget below this widget in the tree.
   final Widget child;
@@ -583,16 +580,15 @@ class AnimatedDefaultTextStyle extends ImplicitlyAnimatedWidget {
   /// Creates a widget that animates the default text style implicitly.
   ///
   /// The [child], [style], [curve], and [duration] arguments must not be null.
-  AnimatedDefaultTextStyle({
+  const AnimatedDefaultTextStyle({
     Key key,
     @required this.child,
     @required this.style,
     Curve curve: Curves.linear,
     @required Duration duration,
-  }) : super(key: key, curve: curve, duration: duration) {
-    assert(style != null);
-    assert(child != null);
-  }
+  }) : assert(style != null),
+       assert(child != null),
+       super(key: key, curve: curve, duration: duration);
 
   /// The widget below this widget in the tree.
   final Widget child;

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

@@ -32,12 +32,11 @@ class LayoutBuilder extends RenderObjectWidget {
   /// Creates a widget that defers its building until layout.
   ///
   /// The [builder] argument must not be null.
-  LayoutBuilder({
+  const LayoutBuilder({
     Key key,
     @required this.builder
-  }) : super(key: key) {
-    assert(builder != null);
-  }
+  }) : assert(builder != null),
+       super(key: key);
 
   /// Called at layout time to construct the widget tree. The builder must not
   /// return null.

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

@@ -12,13 +12,12 @@ class LocaleQueryData { } // TODO(ianh): We need a better type here. This doesn'
 /// Establishes a subtree in which locale queries resolve to the given data.
 class LocaleQuery extends InheritedWidget {
   /// Creates a widget that provides [LocaleQueryData] to its descendants.
-  LocaleQuery({
+  const LocaleQuery({
     Key key,
     @required this.data,
     @required Widget child
-  }) : super(key: key, child: child) {
-    assert(child != null);
-  }
+  }) : assert(child != null),
+       super(key: key, child: child);
 
   /// The locale data for this subtree.
   final LocaleQueryData data;

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

@@ -121,14 +121,13 @@ class MediaQuery extends InheritedWidget {
   /// Creates a widget that provides [MediaQueryData] to its descendants.
   ///
   /// The [data] and [child] arguments must not be null.
-  MediaQuery({
+  const MediaQuery({
     Key key,
     @required this.data,
     @required Widget child,
-  }) : super(key: key, child: child) {
-    assert(child != null);
-    assert(data != null);
-  }
+  }) : assert(child != null),
+       assert(data != null),
+       super(key: key, child: child);
 
   /// Contains information about the current media.
   ///

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

@@ -475,15 +475,14 @@ class Navigator extends StatefulWidget {
   /// Creates a widget that maintains a stack-based history of child widgets.
   ///
   /// The [onGenerateRoute] argument must not be null.
-  Navigator({
+  const Navigator({
     Key key,
     this.initialRoute,
     @required this.onGenerateRoute,
     this.onUnknownRoute,
     this.observers: const <NavigatorObserver>[]
-  }) : super(key: key) {
-    assert(onGenerateRoute != null);
-  }
+  }) : assert(onGenerateRoute != null),
+       super(key: key);
 
   /// The name of the first route to show.
   final String initialRoute;

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

@@ -27,12 +27,11 @@ class OrientationBuilder extends StatelessWidget {
   /// Creates an orientation builder.
   ///
   /// The [builder] argument must not be null.
-  OrientationBuilder({
+  const OrientationBuilder({
     Key key,
     @required this.builder,
-  }) : super(key: key) {
-    assert(builder != null);
-  }
+  }) : assert(builder != null),
+       super(key: key);
 
   /// Builds the widgets below this widget given this widget's orientation.
   final OrientationWidgetBuilder builder;

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

@@ -201,12 +201,11 @@ class Overlay extends StatefulWidget {
   /// Rather than creating an overlay, consider using the overlay that has
   /// already been created by the [WidgetsApp] or the [MaterialApp] for this
   /// application.
-  Overlay({
+  const Overlay({
     Key key,
     this.initialEntries: const <OverlayEntry>[]
-  }) : super(key: key) {
-    assert(initialEntries != null);
-  }
+  }) : assert(initialEntries != null),
+       super(key: key);
 
   /// The entries to include in the overlay initially.
   ///

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

@@ -33,19 +33,18 @@ class GlowingOverscrollIndicator extends StatefulWidget {
   ///
   /// The [showLeading], [showTrailing], [axisDirection], and [color] arguments
   /// must not be null.
-  GlowingOverscrollIndicator({
+  const GlowingOverscrollIndicator({
     Key key,
     this.showLeading: true,
     this.showTrailing: true,
     @required this.axisDirection,
     @required this.color,
     this.child,
-  }) : super(key: key) {
-    assert(showLeading != null);
-    assert(showTrailing != null);
-    assert(axisDirection != null);
-    assert(color != null);
-  }
+  }) : assert(showLeading != null),
+       assert(showTrailing != null),
+       assert(axisDirection != null),
+       assert(color != null),
+       super(key: key);
 
   /// Whether to show the overscroll glow on the side with negative scroll
   /// offsets.

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

@@ -104,13 +104,12 @@ class PageStorage extends StatelessWidget {
   /// Creates a widget that provides a storage bucket for its descendants.
   ///
   /// The [bucket] argument must not be null.
-  PageStorage({
+  const PageStorage({
     Key key,
     @required this.bucket,
     @required this.child
-  }) : super(key: key) {
-    assert(bucket != null);
-  }
+  }) : assert(bucket != null),
+       super(key: key);
 
   /// The widget below this widget in the tree.
   final Widget child;

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

@@ -8,13 +8,12 @@ import 'framework.dart';
 import 'scroll_controller.dart';
 
 class PrimaryScrollController extends InheritedWidget {
-  PrimaryScrollController({
+  const PrimaryScrollController({
     Key key,
     @required this.controller,
     @required Widget child
-  }) : super(key: key, child: child) {
-    assert(controller != null);
-  }
+  }) : assert(controller != null),
+       super(key: key, child: child);
 
   const PrimaryScrollController.none({
     Key key,

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

@@ -28,15 +28,14 @@ class RawKeyboardListener extends StatefulWidget {
   ///
   /// For text entry, consider using a [EditableText], which integrates with
   /// on-screen keyboards and input method editors (IMEs).
-  RawKeyboardListener({
+  const RawKeyboardListener({
     Key key,
     @required this.focusNode,
     @required this.onKey,
     @required this.child,
-  }) : super(key: key) {
-    assert(focusNode != null);
-    assert(child != null);
-  }
+  }) : assert(focusNode != null),
+       assert(child != null),
+       super(key: key);
 
   /// Controls whether this widget has keyboard focus.
   final FocusNode focusNode;

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

@@ -354,18 +354,17 @@ abstract class LocalHistoryRoute<T> extends Route<T> {
 }
 
 class _ModalScopeStatus extends InheritedWidget {
-  _ModalScopeStatus({
+  const _ModalScopeStatus({
     Key key,
     @required this.isCurrent,
     @required this.canPop,
     @required this.route,
     @required Widget child
-  }) : super(key: key, child: child) {
-    assert(isCurrent != null);
-    assert(canPop != null);
-    assert(route != null);
-    assert(child != null);
-  }
+  }) : assert(isCurrent != null),
+       assert(canPop != null),
+       assert(route != null),
+       assert(child != null),
+       super(key: key, child: child);
 
   final bool isCurrent;
   final bool canPop;

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

@@ -25,16 +25,15 @@ export 'package:flutter/physics.dart' show Tolerance;
 typedef Widget ViewportBuilder(BuildContext context, ViewportOffset position);
 
 class Scrollable extends StatefulWidget {
-  Scrollable({
+  const Scrollable({
     Key key,
     this.axisDirection: AxisDirection.down,
     this.controller,
     this.physics,
     @required this.viewportBuilder,
-  }) : super (key: key) {
-    assert(axisDirection != null);
-    assert(viewportBuilder != null);
-  }
+  }) : assert(axisDirection != null),
+       assert(viewportBuilder != null),
+       super (key: key);
 
   final AxisDirection axisDirection;
 
@@ -100,15 +99,14 @@ class Scrollable extends StatefulWidget {
 // Enable Scrollable.of() to work as if ScrollableState was an inherited widget.
 // ScrollableState.build() always rebuilds its _ScrollableScope.
 class _ScrollableScope extends InheritedWidget {
-  _ScrollableScope({
+  const _ScrollableScope({
     Key key,
     @required this.scrollable,
     @required this.position,
     @required Widget child
-  }) : super(key: key, child: child) {
-    assert(scrollable != null);
-    assert(child != null);
-  }
+  }) : assert(scrollable != null),
+       assert(child != null),
+       super(key: key, child: child);
 
   final ScrollableState scrollable;
   final ScrollPosition position;

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

@@ -146,14 +146,13 @@ class SingleChildScrollView extends StatelessWidget {
 }
 
 class _SingleChildViewport extends SingleChildRenderObjectWidget {
-  _SingleChildViewport({
+  const _SingleChildViewport({
     Key key,
     this.axisDirection: AxisDirection.down,
     this.offset,
     Widget child,
-  }) : super(key: key, child: child) {
-    assert(axisDirection != null);
-  }
+  }) : assert(axisDirection != null),
+       super(key: key, child: child);
 
   final AxisDirection axisDirection;
   final ViewportOffset offset;

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

@@ -426,14 +426,13 @@ class SliverGrid extends SliverMultiBoxAdaptorWidget {
 ///    extent in the main axis.
 class SliverFillViewport extends SliverMultiBoxAdaptorWidget {
   /// Creates a sliver whose box children that each fill the viewport.
-  SliverFillViewport({
+  const SliverFillViewport({
     Key key,
     @required SliverChildDelegate delegate,
     this.viewportFraction: 1.0,
-  }) : super(key: key, delegate: delegate) {
-    assert(viewportFraction != null);
-    assert(viewportFraction > 0.0);
-  }
+  }) : assert(viewportFraction != null),
+       assert(viewportFraction > 0.0),
+       super(key: key, delegate: delegate);
 
   /// The fraction of the viewport that each child should fill in the main axis.
   ///

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

@@ -29,16 +29,15 @@ abstract class SliverPersistentHeaderDelegate {
 }
 
 class SliverPersistentHeader extends StatelessWidget {
-  SliverPersistentHeader({
+  const SliverPersistentHeader({
     Key key,
     @required this.delegate,
     this.pinned: false,
     this.floating: false,
-  }) : super(key: key) {
-    assert(delegate != null);
-    assert(pinned != null);
-    assert(floating != null);
-  }
+  }) : assert(delegate != null),
+       assert(pinned != null),
+       assert(floating != null),
+       super(key: key);
 
   final SliverPersistentHeaderDelegate delegate;
 

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

@@ -19,13 +19,12 @@ class TickerMode extends InheritedWidget {
   /// Creates a widget that enables or disables tickers.
   ///
   /// The [enabled] argument must not be null.
-  TickerMode({
+  const TickerMode({
     Key key,
     @required this.enabled,
     Widget child
-  }) : super(key: key, child: child) {
-    assert(enabled != null);
-  }
+  }) : assert(enabled != null),
+       super(key: key, child: child);
 
   /// The current ticker mode of this subtree.
   ///

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

@@ -219,15 +219,14 @@ class SizeTransition extends AnimatedWidget {
   /// The [sizeFactor] argument must not be null. The [axis] argument defaults
   /// to [Axis.vertical]. The [axisAlignment] defaults to 0.5, which centers the
   /// child along the main axis during the transition.
-  SizeTransition({
+  const SizeTransition({
     Key key,
     this.axis: Axis.vertical,
     @required Animation<double> sizeFactor,
     this.axisAlignment: 0.5,
     this.child,
-  }) : super(key: key, listenable: sizeFactor) {
-    assert(axis != null);
-  }
+  }) : assert(axis != null),
+       super(key: key, listenable: sizeFactor);
 
   /// [Axis.horizontal] if [sizeFactor] modifies the width, otherwise [Axis.vertical].
   final Axis axis;
@@ -465,14 +464,13 @@ class AnimatedBuilder extends AnimatedWidget {
   /// Creates an animated builder.
   ///
   /// The [animation] and [builder] arguments must not be null.
-  AnimatedBuilder({
+  const AnimatedBuilder({
     Key key,
     @required Listenable animation,
     @required this.builder,
     this.child,
-  }) : super(key: key, listenable: animation) {
-    assert(builder != null);
-  }
+  }) : assert(builder != null),
+       super(key: key, listenable: animation);
 
   /// Called every time the animation changes value.
   final TransitionBuilder builder;

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

@@ -19,13 +19,12 @@ class WillPopScope extends StatefulWidget {
   /// dismiss the enclosing [ModalRoute].
   ///
   /// The [child] argument must not be null.
-  WillPopScope({
+  const WillPopScope({
     Key key,
     @required this.child,
     @required this.onWillPop,
-  }) : super(key: key) {
-    assert(child != null);
-  }
+  }) : assert(child != null),
+       super(key: key);
 
   /// The widget below this widget in the tree.
   final Widget child;

packages/flutter/test/cupertino/activity_indicator_test.dart

@@ -9,18 +9,18 @@ import 'package:flutter_test/flutter_test.dart';
 
 void main() {
   testWidgets('Activity indicator animate property works', (WidgetTester tester) async {
-    await tester.pumpWidget(new Center(child: new CupertinoActivityIndicator()));
+    await tester.pumpWidget(const Center(child: const CupertinoActivityIndicator()));
     expect(SchedulerBinding.instance.transientCallbackCount, equals(1));
 
-    await tester.pumpWidget(new Center(child: new CupertinoActivityIndicator(animating: false)));
+    await tester.pumpWidget(const Center(child: const CupertinoActivityIndicator(animating: false)));
     expect(SchedulerBinding.instance.transientCallbackCount, equals(0));
 
     await tester.pumpWidget(new Container());
 
-    await tester.pumpWidget(new Center(child: new CupertinoActivityIndicator(animating: false)));
+    await tester.pumpWidget(const Center(child: const CupertinoActivityIndicator(animating: false)));
     expect(SchedulerBinding.instance.transientCallbackCount, equals(0));
 
-    await tester.pumpWidget(new Center(child: new CupertinoActivityIndicator()));
+    await tester.pumpWidget(const Center(child: const CupertinoActivityIndicator()));
     expect(SchedulerBinding.instance.transientCallbackCount, equals(1));
   });
 }

packages/flutter/test/cupertino/button_test.dart

@@ -15,7 +15,7 @@ const TextStyle testStyle = const TextStyle(
 void main() {
   testWidgets('Default layout minimum size', (WidgetTester tester) async {
     await tester.pumpWidget(
-      new Center(child: new CupertinoButton(
+      const Center(child: const CupertinoButton(
         child: const Text('X', style: testStyle),
         onPressed: null,
       ))
@@ -47,7 +47,7 @@ void main() {
 
   testWidgets('Size grows with text', (WidgetTester tester) async {
     await tester.pumpWidget(
-      new Center(child: new CupertinoButton(
+      const Center(child: const CupertinoButton(
         child: const Text('XXXX', style: testStyle),
         onPressed: null,
       ))
@@ -61,7 +61,7 @@ void main() {
   });
 
   testWidgets('Button with background is wider', (WidgetTester tester) async {
-    await tester.pumpWidget(new Center(child: new CupertinoButton(
+    await tester.pumpWidget(const Center(child: const CupertinoButton(
       child: const Text('X', style: testStyle),
       onPressed: null,
       color: const Color(0xFFFFFFFF),
@@ -75,7 +75,7 @@ void main() {
   });
 
   testWidgets('Custom padding', (WidgetTester tester) async {
-    await tester.pumpWidget(new Center(child: new CupertinoButton(
+    await tester.pumpWidget(const Center(child: const CupertinoButton(
       child: const Text(' ', style: testStyle),
       onPressed: null,
       padding: const EdgeInsets.all(100.0),
@@ -116,7 +116,7 @@ void main() {
   });
 
   testWidgets('Disabled button doesn\'t animate', (WidgetTester tester) async {
-    await tester.pumpWidget(new Center(child: new CupertinoButton(
+    await tester.pumpWidget(const Center(child: const CupertinoButton(
       child: const Text('Tap me'),
       onPressed: null,
     )));

packages/flutter/test/cupertino/dialog_test.dart

@@ -23,7 +23,7 @@ void main() {
                       title: const Text('The title'),
                       content: const Text('The content'),
                       actions: <Widget>[
-                        new CupertinoDialogAction(
+                        const CupertinoDialogAction(
                           child: const Text('Cancel'),
                         ),
                         new CupertinoDialogAction(
@@ -64,7 +64,7 @@ void main() {
   });
 
   testWidgets('Dialog action styles', (WidgetTester tester) async {
-    await tester.pumpWidget(new CupertinoDialogAction(
+    await tester.pumpWidget(const CupertinoDialogAction(
       isDestructive: true,
       child: const Text('Ok'),
     ));

packages/flutter/test/material/about_test.dart

@@ -67,7 +67,7 @@ void main() {
 
   testWidgets('About box logic defaults to executable name for app name', (WidgetTester tester) async {
     await tester.pumpWidget(
-      new Material(child: const AboutListTile()),
+      const Material(child: const AboutListTile()),
     );
     expect(find.text('About flutter_tester'), findsOneWidget);
   });

packages/flutter/test/material/back_button_test.dart

@@ -9,10 +9,10 @@ void main() {
   testWidgets('BackButton control test', (WidgetTester tester) async {
     await tester.pumpWidget(
       new MaterialApp(
-        home: new Material(child: const Text('Home')),
+        home: const Material(child: const Text('Home')),
         routes: <String, WidgetBuilder>{
           '/next': (BuildContext context) {
-            return new Material(
+            return const Material(
               child: const Center(
                 child: const BackButton(),
               )

packages/flutter/test/material/debug_test.dart

@@ -7,7 +7,7 @@ import 'package:flutter_test/flutter_test.dart';
 
 void main() {
   testWidgets('debugCheckHasMaterial control test', (WidgetTester tester) async {
-    await tester.pumpWidget(new FlatButton(
+    await tester.pumpWidget(const FlatButton(
       onPressed: null,
       child: const Text('Go'),
     ));

packages/flutter/test/material/dialog_test.dart

@@ -100,7 +100,7 @@ void main() {
   testWidgets('Simple dialog control test', (WidgetTester tester) async {
     await tester.pumpWidget(
       new MaterialApp(
-        home: new Material(
+        home: const Material(
           child: const Center(
             child: const RaisedButton(
               onPressed: null,
@@ -141,7 +141,7 @@ void main() {
   testWidgets('Barrier dismissible', (WidgetTester tester) async {
     await tester.pumpWidget(
       new MaterialApp(
-        home: new Material(
+        home: const Material(
           child: const Center(
             child: const RaisedButton(
               onPressed: null,

packages/flutter/test/material/divider_test.dart

@@ -7,7 +7,7 @@ import 'package:flutter_test/flutter_test.dart';
 
 void main() {
   testWidgets('Divider control test', (WidgetTester tester) async {
-    await tester.pumpWidget(new Center(child: new Divider()));
+    await tester.pumpWidget(const Center(child: const Divider()));
     final RenderBox box = tester.firstRenderObject(find.byType(Divider));
     expect(box.size.height, 15.0);
   });

packages/flutter/test/material/drawer_test.dart

@@ -20,7 +20,7 @@ void main() {
                   child: const Text('header')
                 )
               ),
-              new ListTile(
+              const ListTile(
                 leading: const Icon(Icons.archive),
                 title: const Text('Archive')
               )

packages/flutter/test/material/expand_icon_test.dart

@@ -31,9 +31,9 @@ void main() {
 
   testWidgets('ExpandIcon disabled', (WidgetTester tester) async {
     await tester.pumpWidget(
-      new Material(
-        child: new Center(
-          child: new ExpandIcon(
+      const Material(
+        child: const Center(
+          child: const ExpandIcon(
             onPressed: null
           )
         )

packages/flutter/test/material/grid_title_test.dart

@@ -36,7 +36,7 @@ void main() {
     expect(tester.getBottomLeft(find.byKey(headerKey)).dy,
            lessThan(tester.getTopLeft(find.byKey(footerKey)).dy));
 
-    await tester.pumpWidget(new GridTile(child: const Text('Simple')));
+    await tester.pumpWidget(const GridTile(child: const Text('Simple')));
 
     expect(find.text('Simple'), findsOneWidget);
   });

packages/flutter/test/material/icon_test.dart

@@ -35,8 +35,8 @@ void main() {
 
   testWidgets('Icon sizing - sized theme', (WidgetTester tester) async {
     await tester.pumpWidget(
-      new Center(
-        child: new IconTheme(
+      const Center(
+        child: const IconTheme(
           data: const IconThemeData(size: 36.0),
           child: const Icon(null),
         ),
@@ -49,8 +49,8 @@ void main() {
 
   testWidgets('Icon sizing - sized theme, explicit size', (WidgetTester tester) async {
     await tester.pumpWidget(
-      new Center(
-        child: new IconTheme(
+      const Center(
+        child: const IconTheme(
           data: const IconThemeData(size: 36.0),
           child: const Icon(
             null,
@@ -66,8 +66,8 @@ void main() {
 
   testWidgets('Icon sizing - sizeless theme, default size', (WidgetTester tester) async {
     await tester.pumpWidget(
-      new Center(
-        child: new IconTheme(
+      const Center(
+        child: const IconTheme(
           data: const IconThemeData(),
           child: const Icon(null),
         ),

packages/flutter/test/material/image_icon_test.dart

@@ -27,8 +27,8 @@ void main() {
 
   testWidgets('Icon opacity', (WidgetTester tester) async {
     await tester.pumpWidget(
-      new Center(
-        child: new IconTheme(
+      const Center(
+        child: const IconTheme(
           data: const IconThemeData(opacity: 0.5),
           child: const ImageIcon(_kImage),
         ),
@@ -56,8 +56,8 @@ void main() {
 
   testWidgets('ImageIcon sizing - sized theme', (WidgetTester tester) async {
     await tester.pumpWidget(
-      new Center(
-        child: new IconTheme(
+      const Center(
+        child: const IconTheme(
           data: const IconThemeData(size: 36.0),
           child: const ImageIcon(null)
         )
@@ -70,8 +70,8 @@ void main() {
 
   testWidgets('ImageIcon sizing - sized theme, explicit size', (WidgetTester tester) async {
     await tester.pumpWidget(
-      new Center(
-        child: new IconTheme(
+      const Center(
+        child: const IconTheme(
           data: const IconThemeData(size: 36.0),
           child: const ImageIcon(
             null,
@@ -87,8 +87,8 @@ void main() {
 
   testWidgets('ImageIcon sizing - sizeless theme, default size', (WidgetTester tester) async {
     await tester.pumpWidget(
-      new Center(
-        child: new IconTheme(
+      const Center(
+        child: const IconTheme(
           data: const IconThemeData(),
           child: const ImageIcon(null)
         )

packages/flutter/test/material/page_test.dart

@@ -11,10 +11,10 @@ void main() {
     await tester.pumpWidget(
       new MaterialApp(
         theme: new ThemeData(platform: TargetPlatform.android),
-        home: new Material(child: const Text('Page 1')),
+        home: const Material(child: const Text('Page 1')),
         routes: <String, WidgetBuilder>{
           '/next': (BuildContext context) {
-            return new Material(child: const Text('Page 2'));
+            return const Material(child: const Text('Page 2'));
           },
         },
       )
@@ -62,7 +62,7 @@ void main() {
     await tester.pumpWidget(
       new MaterialApp(
         theme: new ThemeData(platform: TargetPlatform.iOS),
-        home: new Material(child: const Text('Page 1')),
+        home: const Material(child: const Text('Page 1')),
         routes: <String, WidgetBuilder>{
           '/next': (BuildContext context) {
             return new Material(
@@ -141,7 +141,7 @@ void main() {
     await tester.pumpWidget(
       new MaterialApp(
         theme: new ThemeData(platform: TargetPlatform.iOS),
-        home: new Material(child: const Text('Page 1')),
+        home: const Material(child: const Text('Page 1')),
       )
     );
 
@@ -149,7 +149,7 @@ void main() {
 
     tester.state<NavigatorState>(find.byType(Navigator)).push(new MaterialPageRoute<Null>(
       builder: (BuildContext context) {
-        return new Material(child: const Text('Page 2'));
+        return const Material(child: const Text('Page 2'));
       },
       fullscreenDialog: true,
     ));