Fix toolbar height to match Material Design.

54px on small screens:
http://www.google.com/design/spec/layout/metrics-keylines.html#metrics-keylines-keylines-spacing

TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/995133002

examples/fn/widgets/toolbar.dart

@@ -7,7 +7,7 @@ class Toolbar extends Component {
   static final Style _style = new Style('''
     display: flex;
     align-items: center;
-    height: 84px;
+    height: 56px;
     z-index: 1;
     background-color: ${Purple[500]};
     color: white;