Port sky_home to the new SkyView world

sky/sdk/lib/framework/widgets/tool_bar.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import '../rendering/flex.dart';
 import '../theme2/shadows.dart';
 import '../theme2/view_configuration.dart';
 import 'wrappers.dart';
 
 class ToolBar extends Component {
 
   ToolBar({
     String key,
     this.left,
     this.center,
     this.right,
     this.backgroundColor
   }) : super(key: key);
 
   final UINode left;
   final UINode center;
   final List<UINode> right;
   final Color backgroundColor;
 
   UINode build() {
-    List<UINode> children = [
-      left,
-      new FlexExpandingChild(
-        new Padding(
-          child: center,
-          padding: new EdgeDims.only(left: 24.0)
-        ))
-    ];
+    List<UINode> children = new List<UINode>();
+    if (left != null)
+      children.add(left);
+
+    if (center != null) {
+      children.add(new FlexExpandingChild(
+        new Padding(child: center, padding: new EdgeDims.only(left: 24.0))));
+    }
 
     if (right != null)
       children.addAll(right);
 
     return new Container(
       child: new Flex(
         [new Container(child: new Flex(children), height: kToolBarHeight)],
         alignItems: FlexAlignItems.flexEnd
       ),
       padding: new EdgeDims.symmetric(horizontal: 8.0),
       decoration: new BoxDecoration(
         backgroundColor: backgroundColor,
         boxShadow: shadows[2]
       )
     );
   }
 
 }