Roll Observatory packages and add a roll script

packages/quiver/lib/src/streams/enumerate.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.streams;
 
 /**
  * Returns a [Stream] of [IndexedValue]s where the nth value holds the nth
  * element of [stream] and its index.
  */
 Stream<IndexedValue> enumerate(Stream stream) {
   var index = 0;
   return stream.map((value) => new IndexedValue(index++, value));
 }