Roll Observatory packages and add a roll script

packages/charted/lib/core/scales/time_scale.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.core.scales;
 
@@ skipping 165 matching lines @@
         val is int ? val + dt : (val as DateTime).millisecondsSinceEpoch + dt);
   }
 
   List range(var t0, var t1, int step) {
     int start = t0 is DateTime ? t0.millisecondsSinceEpoch : t0,
         stop = t1 is DateTime ? t1.millisecondsSinceEpoch : t1;
     return new Range((start / step).ceil() * step, stop, step).map(
         (d) => new DateTime.fromMillisecondsSinceEpoch(d)).toList();
   }
 }