Roll Observatory packages and add a roll script

packages/which/lib/src/which_impl.dart

 
 library which.src.which_impl;
 
 import 'dart:async';
 
 import 'package:when/when.dart';
 
 import 'util.dart';
 
 Future<String> which(
@@ skipping 32 matching lines @@
         toSequence(candidatePaths),
         (path) => isExecutable(path, isWindows),
         orElse: orElse != null ? orElse : () => _commandNotFound(command, null)),
     onError: (e) => _commandNotFound(command, e));
 
 _commandNotFound(String command, e) {
   var message = 'Command not found: $command';
   if (e != null) message += '\n$e';
   throw new StateError(message);
 }