Roll Observatory packages and add a roll script

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));
-}