Fix toolbar height to match Material Design.

sky/examples/fn/widgets/toolbar.dart

 part of widgets;
 
 class Toolbar extends Component {
 
   List<Node> children;
 
   static final Style _style = new Style('''
     display: flex;
     align-items: center;
-    height: 84px;
+    height: 56px;
     z-index: 1;
     background-color: ${Purple[500]};
     color: white;
     box-shadow: ${Shadow[2]};'''
   );
 
   Toolbar({String key, this.children}) : super(key: key);
 
   Node build() {
     return new Container(
       style: _style,
       children: children
     );
   }
 }