Update Button to be made of Material

sky/examples/stocks-fn/stockarrow.dart

 part of stocksapp;
 
 class StockArrow extends Component {
 
   double percentChange;
 
   static Style _style = new Style('''
     width: 40px;
     height: 40px;
     display: flex;
@@ skipping 56 matching lines @@
     return _kRedColors[_colorIndexForPercentChange(percentChange)];
   }
 
   Node build() {
     String border = _colorForPercentChange(percentChange).toString();
     bool up = percentChange > 0;
     String type = up ? 'bottom' : 'top';
 
     return new Container(
       inlineStyle: 'border-color: $border',
-      style: _style,
+      styles: [_style],
       children: [
         new Container(
           inlineStyle: 'border-$type-color: $border',
-          style: up ? _upStyle : _downStyle
+          styles: [up ? _upStyle : _downStyle]
         )
       ]
     );
   }
 }