Roll Observatory packages and add a roll script

packages/charted/lib/charts/chart_theme.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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   ///
   /// Font string must be of the following form:
   ///   "bold italic 16px Roboto"
   ///   "bold 16px Roboto"
   ///   "italic 16px Roboto"
   ///   "16px Roboto"
   ///
   /// When not specified, SVGTextElement's metrics API will be used.
   String get ticksFont => null;
 }