Roll Observatory packages and add a roll script

packages/charted/lib/charts/behaviors/hovercard.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;
 
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       }
     }
     if (formatter == null) {
       // Formatter function must return String.  Default to identity function
       // but return the toString() instead.
       formatter = (x) => x.toString();
     }
     return formatter;
   }
 }