Roll Observatory packages and add a roll script

packages/quiver/lib/src/iterables/generating_iterable.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,
@@ skipping 60 matching lines @@
   bool moveNext() {
     if (object == null) return false;
     if (started) {
       object = next(object);
     } else {
       started = true;
     }
     return object != null;
   }
 }