Roll Observatory packages and add a roll script

packages/charted/lib/core/time_interval.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
 //
 
 library charted.core.time_intervals;
 
@@ skipping 116 matching lines @@
   static TimeInterval year = new TimeInterval(
       (DateTime date) => new DateTime(date.year),
       (DateTime date, num offset) =>
           new DateTime(date.year + offset, date.month, date.day,
               date.hour, date.minute, date.second, date.millisecond),
       (DateTime date) => date.year);
 
   static int dayOfYear(DateTime date) =>
       date.difference(year.floor(date)).inDays;
 }