Default child lists to empty lists (#6592)

Fixes #6591

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

@@ -203,7 +203,7 @@ class AboutDialog extends StatelessWidget {
     this.applicationVersion,
     this.applicationIcon,
     this.applicationLegalese,
-    this.children
+    this.children,
   }) : super(key: key);
 
   /// The name of the application.

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

@@ -30,7 +30,7 @@ class ButtonBar extends StatelessWidget {
     Key key,
     this.alignment: MainAxisAlignment.end,
     this.mainAxisSize: MainAxisSize.max,
-    this.children
+    this.children: const <Widget>[],
   }) : super(key: key);
 
   /// How the children should be placed along the horizontal axis.

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

@@ -68,7 +68,7 @@ class MaterialList extends StatelessWidget {
     this.onScroll,
     this.onScrollEnd,
     this.type: MaterialListType.twoLine,
-    this.children,
+    this.children: const <Widget>[],
     this.padding: EdgeInsets.zero,
     this.scrollableKey
   }) : super(key: key);

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

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flutter/widgets.dart';
+import 'package:meta/meta.dart';
 
 import 'colors.dart';
 import 'icon.dart';
@@ -32,7 +33,7 @@ class TwoLevelListItem extends StatelessWidget {
   TwoLevelListItem({
     Key key,
     this.leading,
-    this.title,
+    @required this.title,
     this.trailing,
     this.enabled: true,
     this.onTap,
@@ -107,10 +108,10 @@ class TwoLevelSublist extends StatefulWidget {
   TwoLevelSublist({
     Key key,
     this.leading,
-    this.title,
+    @required this.title,
     this.backgroundColor,
     this.onOpenChanged,
-    this.children
+    this.children: const <Widget>[],
   }) : super(key: key);
 
   /// A widget to display before the title.
@@ -263,7 +264,7 @@ class TwoLevelList extends StatelessWidget {
   TwoLevelList({
     Key key,
     this.scrollableKey,
-    this.children,
+    this.children: const <Widget>[],
     this.type: MaterialListType.twoLine,
     this.padding
   }) : super(key: key) {

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

@@ -532,6 +532,10 @@ class RenderGrid extends RenderVirtualViewport<GridParentData> {
 
   /// The delegate that controls the layout of the children.
   ///
+  /// For example, a [FixedColumnCountGridDelegate] for grids that have a fixed
+  /// number of columns or a [MaxTileWidthGridDelegate] for grids that have a
+  /// maximum tile width.
+  ///
   /// If the new delegate is the same as the previous one, this does nothing.
   ///
   /// If the new delegate is the same class as the previous one, then the new

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

@@ -745,8 +745,6 @@ class LayoutId extends ParentDataWidget<CustomMultiChildLayout> {
   }
 }
 
-const List<Widget> _emptyWidgetList = const <Widget>[];
-
 /// A widget that defers the layout of multiple children to a delegate.
 ///
 /// The delegate can determine the layout constraints for each child and can
@@ -767,7 +765,7 @@ class CustomMultiChildLayout extends MultiChildRenderObjectWidget {
   CustomMultiChildLayout({
     Key key,
     @required this.delegate,
-    List<Widget> children: _emptyWidgetList
+    List<Widget> children: const <Widget>[],
   }) : super(key: key, children: children) {
     assert(delegate != null);
   }
@@ -1414,7 +1412,7 @@ class BlockBody extends MultiChildRenderObjectWidget {
   BlockBody({
     Key key,
     this.mainAxis: Axis.vertical,
-    List<Widget> children: _emptyWidgetList
+    List<Widget> children: const <Widget>[],
   }) : super(key: key, children: children) {
     assert(mainAxis != null);
   }
@@ -1468,7 +1466,7 @@ class Stack extends MultiChildRenderObjectWidget {
     Key key,
     this.alignment: FractionalOffset.topLeft,
     this.overflow: Overflow.clip,
-    List<Widget> children: _emptyWidgetList
+    List<Widget> children: const <Widget>[],
   }) : super(key: key, children: children);
 
   /// How to align the non-positioned children in the stack.
@@ -1514,7 +1512,7 @@ class IndexedStack extends Stack {
     Key key,
     FractionalOffset alignment: FractionalOffset.topLeft,
     this.index: 0,
-    List<Widget> children: _emptyWidgetList
+    List<Widget> children: const <Widget>[],
   }) : super(key: key, alignment: alignment, children: children) {
     assert(index != null);
   }
@@ -1710,8 +1708,8 @@ class Positioned extends ParentDataWidget<Stack> {
 abstract class GridRenderObjectWidget extends MultiChildRenderObjectWidget {
   /// Initializes fields for subclasses.
   GridRenderObjectWidget({
-    List<Widget> children: _emptyWidgetList,
-    Key key
+    Key key,
+    List<Widget> children: const <Widget>[],
   }) : super(key: key, children: children) {
     _delegate = createDelegate();
   }
@@ -1743,12 +1741,16 @@ class CustomGrid extends GridRenderObjectWidget {
   CustomGrid({
     Key key,
     @required this.delegate,
-    List<Widget> children: _emptyWidgetList
+    List<Widget> children: const <Widget>[],
   }) : super(key: key, children: children) {
     assert(delegate != null);
   }
 
   /// The delegate that controls the layout of the children.
+  ///
+  /// For example, a [FixedColumnCountGridDelegate] for grids that have a fixed
+  /// number of columns or a [MaxTileWidthGridDelegate] for grids that have a
+  /// maximum tile width.
   final GridDelegate delegate;
 
   @override
@@ -1769,7 +1771,7 @@ class FixedColumnCountGrid extends GridRenderObjectWidget {
     this.rowSpacing: 0.0,
     this.tileAspectRatio: 1.0,
     this.padding: EdgeInsets.zero,
-    List<Widget> children: _emptyWidgetList
+    List<Widget> children: const <Widget>[],
   }) : super(key: key, children: children) {
     assert(columnCount != null && columnCount >= 0);
     assert(tileAspectRatio != null && tileAspectRatio > 0.0);
@@ -1816,7 +1818,7 @@ class MaxTileWidthGrid extends GridRenderObjectWidget {
     this.rowSpacing: 0.0,
     this.tileAspectRatio: 1.0,
     this.padding: EdgeInsets.zero,
-    List<Widget> children: _emptyWidgetList
+    List<Widget> children: const <Widget>[],
   }) : super(key: key, children: children) {
     assert(maxTileWidth != null && maxTileWidth >= 0.0);
     assert(tileAspectRatio != null && tileAspectRatio > 0.0);
@@ -1945,7 +1947,7 @@ class Flex extends MultiChildRenderObjectWidget {
     this.mainAxisSize: MainAxisSize.max,
     this.crossAxisAlignment: CrossAxisAlignment.center,
     this.textBaseline,
-    List<Widget> children: _emptyWidgetList
+    List<Widget> children: const <Widget>[],
   }) : super(key: key, children: children) {
     assert(direction != null);
     assert(mainAxisAlignment != null);
@@ -2047,7 +2049,7 @@ class Row extends Flex {
     MainAxisSize mainAxisSize: MainAxisSize.max,
     CrossAxisAlignment crossAxisAlignment: CrossAxisAlignment.center,
     TextBaseline textBaseline,
-    List<Widget> children: _emptyWidgetList
+    List<Widget> children: const <Widget>[],
   }) : super(
     children: children,
     key: key,
@@ -2101,7 +2103,7 @@ class Column extends Flex {
     MainAxisSize mainAxisSize: MainAxisSize.max,
     CrossAxisAlignment crossAxisAlignment: CrossAxisAlignment.center,
     TextBaseline textBaseline,
-    List<Widget> children: _emptyWidgetList
+    List<Widget> children: const <Widget>[],
   }) : super(
     children: children,
     key: key,
@@ -2212,7 +2214,7 @@ class Flow extends MultiChildRenderObjectWidget {
   Flow({
     Key key,
     @required this.delegate,
-    List<Widget> children: _emptyWidgetList
+    List<Widget> children: const <Widget>[],
   }) : super(key: key, children: RepaintBoundary.wrapAll(children)) {
     assert(delegate != null);
   }
@@ -2227,7 +2229,7 @@ class Flow extends MultiChildRenderObjectWidget {
   Flow.unwrapped({
     Key key,
     this.delegate,
-    List<Widget> children: _emptyWidgetList
+    List<Widget> children: const <Widget>[],
   }) : super(key: key, children: children) {
     assert(delegate != null);
   }

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

@@ -1215,7 +1215,7 @@ abstract class MultiChildRenderObjectWidget extends RenderObjectWidget {
   ///
   /// The [children] argument must not be null and must not contain any null
   /// objects.
-  MultiChildRenderObjectWidget({ Key key, this.children })
+  MultiChildRenderObjectWidget({ Key key, this.children: const <Widget>[] })
     : super(key: key) {
     assert(children != null);
     assert(!children.any((Widget child) => child == null));

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

@@ -144,7 +144,7 @@ class LazyBlockChildren extends LazyBlockDelegate {
   ///
   /// The list of children must not be modified after being passed to this
   /// constructor.
-  LazyBlockChildren({ this.children }) {
+  LazyBlockChildren({ this.children: const <Widget>[] }) {
     assert(children != null);
   }
 

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

@@ -122,7 +122,7 @@ class PageableList extends Pageable {
     ValueChanged<int> onPageChanged,
     Duration duration: const Duration(milliseconds: 200),
     Curve curve: Curves.ease,
-    this.children
+    this.children: const <Widget>[],
   }) : super(
     key: key,
     initialScrollOffset: initialScrollOffset,
@@ -542,7 +542,7 @@ class PageViewport extends _VirtualPageViewport with VirtualViewportFromIterable
     Axis mainAxis: Axis.vertical,
     ViewportAnchor anchor: ViewportAnchor.start,
     bool itemsWrap: false,
-    this.children
+    this.children: const <Widget>[],
   }) : super(
     startOffset,
     mainAxis,

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

@@ -35,7 +35,7 @@ class ScrollableGrid extends StatelessWidget {
     this.snapOffsetCallback,
     this.scrollableKey,
     @required this.delegate,
-    this.children
+    this.children: const <Widget>[],
   }) : super(key: key) {
     assert(delegate != null);
   }
@@ -75,6 +75,10 @@ class ScrollableGrid extends StatelessWidget {
   final Key scrollableKey;
 
   /// The delegate that controls the layout of the children.
+  ///
+  /// For example, a [FixedColumnCountGridDelegate] for grids that have a fixed
+  /// number of columns or a [MaxTileWidthGridDelegate] for grids that have a
+  /// maximum tile width.
   final GridDelegate delegate;
 
   /// The children that will be placed in the grid.
@@ -124,7 +128,7 @@ class GridViewport extends VirtualViewportFromIterable {
     this.scrollOffset,
     this.delegate,
     this.onExtentsChanged,
-    this.children
+    this.children: const <Widget>[],
   }) {
     assert(delegate != null);
   }
@@ -140,6 +144,10 @@ class GridViewport extends VirtualViewportFromIterable {
   }
 
   /// The delegate that controls the layout of the children.
+  ///
+  /// For example, a [FixedColumnCountGridDelegate] for grids that have a fixed
+  /// number of columns or a [MaxTileWidthGridDelegate] for grids that have a
+  /// maximum tile width.
   final GridDelegate delegate;
 
   /// Called when the interior or exterior dimensions of the viewport change.

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

@@ -49,7 +49,7 @@ class ScrollableList extends StatelessWidget {
     @required this.itemExtent,
     this.itemsWrap: false,
     this.padding,
-    this.children
+    this.children: const <Widget>[],
   }) : super(key: key) {
     assert(scrollDirection != null);
     assert(scrollAnchor != null);
@@ -389,7 +389,7 @@ class ListViewport extends _VirtualListViewport with VirtualViewportFromIterable
     @required double itemExtent,
     bool itemsWrap: false,
     EdgeInsets padding,
-    this.children
+    this.children: const <Widget>[],
   }) : super(
     onExtentsChanged,
     scrollOffset,

packages/flutter/test/material/button_bar_test.dart

@@ -0,0 +1,12 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('ButtonBar default control', (WidgetTester tester) async {
+    await tester.pumpWidget(new Center(child: new ButtonBar()));
+  });
+}

packages/flutter/test/material/list_test.dart

@@ -0,0 +1,12 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('MaterialList default control', (WidgetTester tester) async {
+    await tester.pumpWidget(new Center(child: new MaterialList()));
+  });
+}

packages/flutter/test/material/two_level_list_test.dart

@@ -7,7 +7,25 @@ import 'package:flutter/widgets.dart';
 import 'package:flutter/material.dart';
 
 void main() {
-  testWidgets('TwoLeveList basics', (WidgetTester tester) async {
+  testWidgets('TwoLevelList default control', (WidgetTester tester) async {
+    await tester.pumpWidget(new Center(child: new TwoLevelList()));
+
+    await tester.pumpWidget(
+      new Material(
+        child: new Center(
+          child: new TwoLevelList(
+            children: <Widget>[
+              new TwoLevelSublist(
+                title: new Text('Title'),
+              )
+            ]
+          )
+        )
+      )
+    );
+  });
+
+  testWidgets('TwoLevelList basics', (WidgetTester tester) async {
     final Key topKey = new UniqueKey();
     final Key sublistKey = new UniqueKey();
     final Key bottomKey = new UniqueKey();

packages/flutter/test/widget/pageable_list_test.dart

@@ -60,6 +60,10 @@ Future<Null> pageRight(WidgetTester tester) {
 }
 
 void main() {
+  testWidgets('PageableList default control', (WidgetTester tester) async {
+    await tester.pumpWidget(new Center(child: new PageableList()));
+  });
+
   testWidgets('PageableList with itemsWrap: false', (WidgetTester tester) async {
     currentPage = null;
     await tester.pumpWidget(buildFrame());

packages/flutter/test/widget/scrollable_grid_test.dart

@@ -7,6 +7,12 @@ import 'package:flutter/widgets.dart';
 import 'package:flutter/rendering.dart';
 
 void main() {
+  testWidgets('ScrollableGrid default control', (WidgetTester tester) async {
+    await tester.pumpWidget(new Center(child: new ScrollableGrid(
+      delegate: new FixedColumnCountGridDelegate(columnCount: 1),
+    )));
+  });
+
   // Tests https://github.com/flutter/flutter/issues/5522
   testWidgets('ScrollableGrid displays correct children with nonzero padding', (WidgetTester tester) async {
     GlobalKey<ScrollableState> scrollableKey = new GlobalKey<ScrollableState>();

packages/flutter/test/widget/scrollable_list_test.dart

@@ -0,0 +1,12 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('ScrollableList default control', (WidgetTester tester) async {
+    await tester.pumpWidget(new Center(child: new ScrollableList(itemExtent: 100.0)));
+  });
+}