Add a constant for padding in a Material ListView (#8918)

Fixes #8235

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

@@ -84,7 +84,7 @@ class MenuDemoState extends State<MenuDemo> {
         ]
       ),
       body: new ListView(
-        padding: const EdgeInsets.symmetric(vertical: 8.0),
+        padding: kMaterialListPadding,
         children: <Widget>[
           // Pressing the PopupMenuButton on the right of this item shows
           // a simple menu with one disabled item. Typically the contents

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

@@ -60,7 +60,7 @@ class OverscrollDemoState extends State<OverscrollDemo> {
         key: _refreshIndicatorKey,
         onRefresh: _handleRefresh,
         child: new ListView.builder(
-          padding: const EdgeInsets.all(8.0),
+          padding: kMaterialListPadding,
           itemCount: _items.length,
           itemBuilder: (BuildContext context, int index) {
             final String item = _items[index];

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

@@ -59,7 +59,7 @@ class ShrinePageState extends State<ShrinePage> {
         );
       }
       return new ListView(
-        padding: const EdgeInsets.symmetric(vertical: 8.0),
+        padding: kMaterialListPadding,
         children: config.shoppingCart.values.map((Order order) {
           return new ListTile(
             title: new Text(order.product.name),

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

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/painting.dart';
+
 /// The height of the toolbar component of the [AppBar].
 const double kToolbarHeight = 56.0;
 
@@ -25,3 +27,5 @@ const int kRadialReactionAlpha = 0x33;
 
 /// The duration
 const Duration kTabScrollDuration = const Duration(milliseconds: 200);
+
+const EdgeInsets kMaterialListPadding = const EdgeInsets.symmetric(vertical: 8.0);

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

@@ -9,6 +9,7 @@ import 'package:flutter/scheduler.dart';
 import 'package:flutter/widgets.dart';
 
 import 'colors.dart';
+import 'constants.dart';
 import 'debug.dart';
 import 'icon.dart';
 import 'icons.dart';
@@ -21,7 +22,6 @@ import 'theme.dart';
 const Duration _kDropdownMenuDuration = const Duration(milliseconds: 300);
 const double _kMenuItemHeight = 48.0;
 const double _kDenseButtonHeight = 24.0;
-const EdgeInsets _kMenuVerticalPadding = const EdgeInsets.symmetric(vertical: 8.0);
 const EdgeInsets _kMenuHorizontalPadding = const EdgeInsets.symmetric(horizontal: 16.0);
 
 class _DropdownMenuPainter extends CustomPainter {
@@ -49,7 +49,7 @@ class _DropdownMenuPainter extends CustomPainter {
 
   @override
   void paint(Canvas canvas, Size size) {
-    final double selectedItemOffset = selectedIndex * _kMenuItemHeight + _kMenuVerticalPadding.top;
+    final double selectedItemOffset = selectedIndex * _kMenuItemHeight + kMaterialListPadding.top;
     final Tween<double> top = new Tween<double>(
       begin: selectedItemOffset.clamp(0.0, size.height - _kMenuItemHeight),
       end: 0.0,
@@ -178,7 +178,7 @@ class _DropdownMenuState<T> extends State<_DropdownMenu<T>> {
             child: new Scrollbar(
               child: new ListView(
                 controller: config.route.scrollController,
-                padding: _kMenuVerticalPadding,
+                padding: kMaterialListPadding,
                 itemExtent: _kMenuItemHeight,
                 shrinkWrap: true,
                 children: children,
@@ -219,7 +219,7 @@ class _DropdownMenuRouteLayout<T> extends SingleChildLayoutDelegate {
   @override
   Offset getPositionForChild(Size size, Size childSize) {
     final double buttonTop = buttonRect.top;
-    final double selectedItemOffset = selectedIndex * _kMenuItemHeight + _kMenuVerticalPadding.top;
+    final double selectedItemOffset = selectedIndex * _kMenuItemHeight + kMaterialListPadding.top;
     double top = (buttonTop - selectedItemOffset) - (_kMenuItemHeight - buttonRect.height) / 2.0;
     final double topPreferredLimit = _kMenuItemHeight;
     if (top < topPreferredLimit)