Roll Observatory packages and add a roll script

packages/charted/lib/charts/behaviors/mouse_tracker.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 100 matching lines @@
         if (_showMarkerX) {
           _markerX.attributes['transform'] = 'translate(${e.chartX},0)';
         }
         if (_showMarkerY) {
           _markerY.attributes['transform'] = 'translate(0,${e.chartY})';
         }
       });
     }
   }
 }