Remove Material's opinion about background colours, since in fact the background colour varies dram…

sky/sdk/lib/widgets/scrollable.dart

 // Copyright 2015 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 'dart:math' as math;
 import 'dart:sky' as sky;
 
 import '../animation/generators.dart';
 import '../animation/mechanics.dart';
 import '../animation/scroll_behavior.dart';
+import '../theme/colors.dart' as colors;
+import '../theme/theme_data.dart';
 import '../theme/view_configuration.dart' as config;
 import 'basic.dart';
 import 'material.dart';
+import 'theme.dart';
 
 const double _kMillisecondsPerSecond = 1000.0;
 
 double _velocityForFlingGesture(sky.GestureEvent event) {
   return math.max(-config.kMaxFlingVelocity, math.min(config.kMaxFlingVelocity,
       -event.velocityY)) / _kMillisecondsPerSecond;
 }
 
 abstract class ScrollClient {
   bool ancestorScrolled(Scrollable ancestor);
 }
 
 abstract class Scrollable extends Component {
 
   Scrollable({ String key, Color this.backgroundColor })
     : super(key: key, stateful: true);
 
   Color backgroundColor;
 
   void syncFields(Scrollable source) {
     backgroundColor = source.backgroundColor;
   }
 
+  Color get _nonNullBackgroundColor {
+    if (backgroundColor != null)
+      return backgroundColor;
+    switch (Theme.of(this).brightness) {
+      case ThemeBrightness.light:
+        return colors.Grey[50];
+      case ThemeBrightness.dark:
+        return colors.Grey[850];
+    }
+  }
+
   double _scrollOffset = 0.0;
   double get scrollOffset => _scrollOffset;
 
   ScrollBehavior _scrollBehavior;
   ScrollBehavior createScrollBehavior();
   ScrollBehavior get scrollBehavior {
     if (_scrollBehavior == null)
       _scrollBehavior = createScrollBehavior();
     return _scrollBehavior;
   }
 
   Simulation _simulation;
 
   Widget buildContent();
 
   Widget build() {
     return new Listener(
       child: new Material(
         child: buildContent(),
         edge: MaterialEdge.canvas,
-        color: backgroundColor
+        color: _nonNullBackgroundColor
       ),
       onPointerDown: _handlePointerDown,
       onPointerUp: _handlePointerUpOrCancel,
       onPointerCancel: _handlePointerUpOrCancel,
       onGestureFlingStart: _handleFlingStart,
       onGestureFlingCancel: _handleFlingCancel,
       onGestureScrollUpdate: _handleScrollUpdate,
       onWheel: _handleWheel
     );
   }
@@ skipping 86 matching lines @@
 
   void _handleFlingCancel(sky.GestureEvent event) {
     _startSimulation(_createParticle());
   }
 
   void _handleWheel(sky.WheelEvent event) {
     scrollBy(-event.offsetY);
   }
 
 }