Roll Observatory packages and add a roll script

packages/quiver/lib/src/time/duration_unit_constants.dart

 // Copyright 2014 Google Inc. All Rights Reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
 part of quiver.time;
 
 const Duration aMicrosecond = const Duration(microseconds: 1);
 const Duration aMillisecond = const Duration(milliseconds: 1);
 const Duration aSecond = const Duration(seconds: 1);
 const Duration aMinute = const Duration(minutes: 1);
 const Duration anHour = const Duration(hours: 1);
 const Duration aDay = const Duration(days: 1);
 const Duration aWeek = const Duration(days: 7);