Add constants for material colors and shadows

sky/examples/fn/widgets/drawer.dart

 part of widgets;
 
 const double _kWidth = 256.0;
 const double _kMinFlingVelocity = 0.4;
 const double _kMinAnimationDurationMS = 246.0;
 const double _kMaxAnimationDurationMS = 600.0;
 const Cubic _kAnimationCurve = easeOut;
 
 class DrawerAnimation {
 
@@ skipping 85 matching lines @@
 
 class Drawer extends Component {
 
   static Style _style = new Style('''
     position: absolute;
     z-index: 2;
     top: 0;
     left: 0;
     bottom: 0;
     right: 0;
-    box-shadow: 0 10px 20px rgba(0,0,0,0.19), 0 6px 6px rgba(0,0,0,0.23);'''
+    box-shadpw: ${Shadow[3]};'''
   );
 
   static Style _maskStyle = new Style('''
     background-color: black;
     will-change: opacity;
     position: absolute;
     top: 0;
     left: 0;
     bottom: 0;
     right: 0;'''
   );
 
   static Style _contentStyle = new Style('''
-    background-color: #FAFAFA;
+    background-color: ${Grey[50]};
     will-change: transform;
     position: absolute;
     z-index: 3;
     width: 256px;
     top: 0;
     left: 0;
     bottom: 0;'''
   );
 
   DrawerAnimation animation;
@@ skipping 47 matching lines @@
       style: _style,
       inlineStyle: inlineStyle,
       children: [ mask, content ]
     )..events.listen('pointerdown', animation.handlePointerDown)
      ..events.listen('pointermove', animation.handlePointerMove)
      ..events.listen('pointerup', animation.handlePointerUp)
      ..events.listen('pointercancel', animation.handlePointerCancel);
 
   }
 }