Add constants for material colors and shadows

sky/examples/fn/widgets/drawerheader.dart

 part of widgets;
 
 class DrawerHeader extends Component {
 
   static Style _style = new Style('''
     display: flex;
     flex-direction: column;
     height: 140px;
     -webkit-user-select: none;
-    background-color: #E3ECF5;
+    background-color: ${BlueGrey[50]};
     border-bottom: 1px solid #D1D9E1;
     padding-bottom: 7px;
     margin-bottom: 8px;'''
   );
 
   static Style _spacerStyle = new Style('''
     flex: 1'''
   );
 
   static Style _labelStyle = new Style('''
@@ skipping 16 matching lines @@
         ),
         new Container(
           key: 'Label',
           style: _labelStyle,
           children: children
         )
       ]
     );
   }
 }