Apply @required according to the rules, for all of widgets/* (#7730)

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

@@ -45,8 +45,8 @@ class AnimatedCrossFade extends StatefulWidget {
   /// order to have finer control, e.g., creating an overlap between the fades.
   AnimatedCrossFade({
     Key key,
-    this.firstChild,
-    this.secondChild,
+    @required this.firstChild,
+    @required this.secondChild,
     this.firstCurve: Curves.linear,
     this.secondCurve: Curves.linear,
     this.sizeCurve: Curves.linear,

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

@@ -168,7 +168,7 @@ class CheckedModeBanner extends StatelessWidget {
   /// Creates a checked mode banner.
   CheckedModeBanner({
     Key key,
-    this.child
+    @required this.child
   }) : super(key: key);
 
   /// The widget to show behind the banner.

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

@@ -91,7 +91,7 @@ class Container extends StatelessWidget {
     BoxConstraints constraints,
     this.margin,
     this.transform,
-    this.child
+    this.child,
   }) : constraints =
         (width != null || height != null)
           ? constraints?.tighten(width: width, height: height)

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

@@ -73,7 +73,7 @@ class Dismissable extends StatefulWidget {
   /// according to their keys and avoids this pitfall.
   Dismissable({
     @required Key key,
-    this.child,
+    @required this.child,
     this.background,
     this.secondaryBackground,
     this.onResize,

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

@@ -24,7 +24,7 @@ class _FocusScope extends InheritedWidget {
     this.scopeFocused,
     this.focusedScope,
     this.focusedWidget,
-    Widget child
+    @required Widget child,
   }) : super(key: key, child: child) {
     assert(scopeFocused != null);
   }
@@ -113,7 +113,7 @@ class Focus extends StatefulWidget {
   Focus({
     @required GlobalKey key,
     this.initiallyFocusedScope,
-    this.child
+    @required this.child,
   }) : super(key: key) {
     assert(key != null);
   }

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

@@ -1071,7 +1071,7 @@ abstract class State<T extends StatefulWidget> {
 /// [ParentDataWidget].
 abstract class ProxyWidget extends Widget {
   /// Creates a widget that has exactly one child widget.
-  const ProxyWidget({ Key key, this.child }) : super(key: key);
+  const ProxyWidget({ Key key, @required this.child }) : super(key: key);
 
   /// The widget below this widget in the tree.
   final Widget child;

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

@@ -375,7 +375,7 @@ class AnimatedPositioned extends ImplicitlyAnimatedWidget {
   /// The [curve] and [duration] arguments must not be null.
   AnimatedPositioned({
     Key key,
-    this.child,
+    @required this.child,
     this.left,
     this.top,
     this.right,

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

@@ -72,7 +72,7 @@ class NotificationListener<T extends Notification> extends StatelessWidget {
   /// Creates a widget that listens for notifications.
   NotificationListener({
     Key key,
-    this.child,
+    @required this.child,
     this.onNotification
   }) : super(key: key);
 

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

@@ -107,7 +107,7 @@ class PageStorage extends StatelessWidget {
   PageStorage({
     Key key,
     @required this.bucket,
-    this.child
+    @required this.child
   }) : super(key: key) {
     assert(bucket != null);
   }

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

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 
 import 'basic.dart';
@@ -30,7 +31,7 @@ class RawKeyboardListener extends StatefulWidget {
     Key key,
     this.focused: false,
     this.onKey,
-    this.child
+    @required this.child,
   }) : super(key: key) {
     assert(child != null);
   }
@@ -98,7 +99,5 @@ class _RawKeyboardListenerState extends State<RawKeyboardListener> {
   }
 
   @override
-  Widget build(BuildContext context) {
-    return config.child;
-  }
+  Widget build(BuildContext context) => config.child;
 }

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

@@ -333,7 +333,7 @@ class _ModalScopeStatus extends InheritedWidget {
     Key key,
     this.isCurrent,
     this.route,
-    Widget child
+    @required Widget child
   }) : super(key: key, child: child) {
     assert(isCurrent != null);
     assert(route != null);
@@ -366,7 +366,7 @@ class _ModalScope extends StatefulWidget {
   _ModalScope({
     Key key,
     this.route,
-    this.page
+    @required this.page,
   }) : super(key: key);
 
   final ModalRoute<dynamic> route;

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

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
 
@@ -12,7 +13,7 @@ class Title extends StatelessWidget {
     Key key,
     this.title,
     this.color,
-    this.child
+    @required this.child,
   }) : super(key: key) {
     assert(color == null || color.alpha == 0xFF);
   }

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

@@ -91,7 +91,7 @@ class SlideTransition extends AnimatedWidget {
     Key key,
     Animation<FractionalOffset> position,
     this.transformHitTests: true,
-    this.child
+    this.child,
   }) : super(key: key, animation: position);
 
   /// The animation that controls the position of the child.
@@ -116,7 +116,7 @@ class SlideTransition extends AnimatedWidget {
     return new FractionalTranslation(
       translation: position.value,
       transformHitTests: transformHitTests,
-      child: child
+      child: child,
     );
   }
 }
@@ -131,7 +131,7 @@ class ScaleTransition extends AnimatedWidget {
     Key key,
     Animation<double> scale,
     this.alignment: FractionalOffset.center,
-    this.child
+    this.child,
   }) : super(key: key, animation: scale);
 
   /// The animation that controls the scale of the child.
@@ -158,7 +158,7 @@ class ScaleTransition extends AnimatedWidget {
     return new Transform(
       transform: transform,
       alignment: alignment,
-      child: child
+      child: child,
     );
   }
 }
@@ -171,7 +171,7 @@ class RotationTransition extends AnimatedWidget {
   RotationTransition({
     Key key,
     Animation<double> turns,
-    this.child
+    this.child,
   }) : super(key: key, animation: turns);
 
   /// The animation that controls the rotation of the child.
@@ -190,7 +190,7 @@ class RotationTransition extends AnimatedWidget {
     return new Transform(
       transform: transform,
       alignment: FractionalOffset.center,
-      child: child
+      child: child,
     );
   }
 }
@@ -210,7 +210,7 @@ class SizeTransition extends AnimatedWidget {
     this.axis: Axis.vertical,
     Animation<double> sizeFactor,
     this.axisAlignment: 0.5,
-    this.child
+    this.child,
   }) : super(key: key, animation: sizeFactor) {
     assert(axis != null);
   }
@@ -241,7 +241,7 @@ class SizeTransition extends AnimatedWidget {
         alignment: alignment,
         heightFactor: axis == Axis.vertical ? sizeFactor.value : null,
         widthFactor: axis == Axis.horizontal ? sizeFactor.value : null,
-        child: child
+        child: child,
       )
     );
   }
@@ -258,7 +258,7 @@ class FadeTransition extends AnimatedWidget {
   FadeTransition({
     Key key,
     Animation<double> opacity,
-    this.child
+    this.child,
   }) : super(key: key, animation: opacity);
 
   /// The animation that controls the opacity of the child.
@@ -310,7 +310,7 @@ class PositionedTransition extends AnimatedWidget {
   PositionedTransition({
     Key key,
     Animation<RelativeRect> rect,
-    this.child
+    @required this.child,
   }) : super(key: key, animation: rect);
 
   /// The animation that controls the child's size and position.
@@ -326,7 +326,7 @@ class PositionedTransition extends AnimatedWidget {
       right: rect.value.right,
       bottom: rect.value.bottom,
       left: rect.value.left,
-      child: child
+      child: child,
     );
   }
 }
@@ -350,7 +350,7 @@ class RelativePositionedTransition extends AnimatedWidget {
     Key key,
     @required Animation<Rect> rect,
     @required this.size,
-    this.child
+    @required this.child,
   }) : super(key: key, animation: rect);
 
   /// The animation that controls the child's size and position.
@@ -373,7 +373,7 @@ class RelativePositionedTransition extends AnimatedWidget {
       right: offsets.right,
       bottom: offsets.bottom,
       left: offsets.left,
-      child: child
+      child: child,
     );
   }
 }
@@ -412,7 +412,7 @@ class AnimatedBuilder extends AnimatedWidget {
     Key key,
     @required Animation<Object> animation,
     @required this.builder,
-    this.child
+    this.child,
   }) : super(key: key, animation: animation) {
     assert(builder != null);
   }

packages/flutter/test/widgets/title_test.dart

@@ -10,6 +10,7 @@ void main() {
     Widget widget = new Title(
       color: const Color(0xFF00FF00),
       title: 'Awesome app',
+      child: new Container(),
     );
     expect(widget.toString, isNot(throwsException));
   });