Roll Observatory packages and add a roll script

packages/which/lib/src/util.dart

 
 library which.src.util;
 
 import 'dart:async';
 
 /// Transparently call `firstWhere` on a [Stream] or [Iterable].
 // TODO: Remove once https://dartbug.com/ is fixed.
 firstWhere(sequence, test, { orElse() }) => sequence is Iterable ?
   sequence.firstWhere(test, orElse: orElse) :
     _streamFirstWhere(sequence, test, orElse: orElse);
 
 Future _streamFirstWhere(Stream stream, test(item), { orElse() }) {
   var pairs = stream.asyncMap((item) => test(item).then((result) => [item, result]));
   return pairs.firstWhere((pair) => pair.last, defaultValue: () => [orElse(), null]).then((pair) => pair.first);
 }
 
 /// The identity function simply returns its argument ([x]).
 identity(x) => x;