Roll Observatory packages and add a roll script

packages/charted/lib/charts/cartesian_renderers/stackedbar_chart_renderer.dart

 //
 // Copyright 2014 Google Inc. All rights reserved.
 //
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file or at
 // https://developers.google.com/open-source/licenses/bsd
 //
 
 part of charted.charts;
 
@@ skipping 306 matching lines @@
     controller.add(new DefaultChartEventImpl(
         scope.event, area, series, row,
         series.measures.elementAt(_reverseIdx(index)), data));
   }
 
   // Stacked bar chart renders items from bottom to top (first measure is at
   // the bottom of the stack). We use [_reversedIdx] instead of index to
   // match the color and order of what is displayed in the legend.
   int _reverseIdx(int index) => series.measures.length - 1 - index;
 }