Make center part of toolbar optional.

packages/flutter/lib/widgets/tool_bar.dart

diff --git a/packages/flutter/lib/widgets/tool_bar.dart b/packages/flutter/lib/widgets/tool_bar.dart
index 20080e3..55beb63 100644
--- a/packages/flutter/lib/widgets/tool_bar.dart
+++ b/packages/flutter/lib/widgets/tool_bar.dart
@@ -52,7 +52,7 @@
     children.add(
       new Flexible(
         child: new Padding(
-          child: new DefaultTextStyle(child: center, style: centerStyle),
+          child: center != null ? new DefaultTextStyle(child: center, style: centerStyle) : null,
           padding: new EdgeDims.only(left: 24.0)
         )
       )