add @required (#9579)

examples/flutter_gallery/lib/demo/colors_demo.dart

@@ -81,7 +81,10 @@ class PaletteTabView extends StatelessWidget {
   static const List<int> primaryKeys = const <int>[50, 100, 200, 300, 400, 500, 600, 700, 800, 900];
   static const List<int> accentKeys = const <int>[100, 200, 400, 700];
 
-  PaletteTabView({ Key key, this.colors }) : super(key: key) {
+  PaletteTabView({
+    Key key,
+    @required this.colors,
+  }) : super(key: key) {
     assert(colors != null && colors.isValid);
   }
 

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

@@ -325,7 +325,7 @@ class _OrderPageState extends State<OrderPage> {
 // final value of the order field.
 class ShrineOrderRoute extends ShrinePageRoute<Order> {
   ShrineOrderRoute({
-    this.order,
+    @required this.order,
     WidgetBuilder builder,
     RouteSettings settings: const RouteSettings(),
   }) : super(builder: builder, settings: settings) {

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

@@ -4,6 +4,7 @@
 
 import 'dart:math' as math;
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 
 import 'basic.dart';
@@ -177,7 +178,7 @@ class _SingleChildViewport extends SingleChildRenderObjectWidget {
 class _RenderSingleChildViewport extends RenderBox with RenderObjectWithChildMixin<RenderBox> implements RenderAbstractViewport {
   _RenderSingleChildViewport({
     AxisDirection axisDirection: AxisDirection.down,
-    ViewportOffset offset,
+    @required ViewportOffset offset,
     RenderBox child,
   }) : _axisDirection = axisDirection,
        _offset = offset {

packages/flutter_tools/lib/src/commands/build_apk.dart

@@ -16,7 +16,12 @@ import 'build.dart';
 export '../android/android_device.dart' show AndroidDevice;
 
 class ApkKeystoreInfo {
-  ApkKeystoreInfo({ this.keystore, this.password, this.keyAlias, @required this.keyPassword }) {
+  ApkKeystoreInfo({
+    @required this.keystore,
+    this.password,
+    this.keyAlias,
+    @required this.keyPassword,
+  }) {
     assert(keystore != null);
   }