Roll Observatory packages and add a roll script

packages/which/lib/which.dart

 
 library which;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'src/candidate_paths.dart';
 import 'src/is_executable.dart';
 import 'src/which_impl.dart' as impl;
 
 /// Returns a future for the first [command] executable in the `PATH`.
 ///
 /// If [command] is not found, [orElse] is called, which defaults to throwing.
 Future<String> which(String command, { orElse() }) => new Future(() => impl.which(
     command,
     getRealCandidatePaths(command),
     Platform.isWindows,
     (path, isWindows) => isExecutable(path, isWindows, FileStat.stat),
     orElse));
 
 /// Returns the first [command] executable in the `PATH`.
 ///
 /// If [command] is not found, [orElse] is called, which defaults to throwing.
 String whichSync(String command, { orElse() }) => impl.whichSync(
     command,
     getRealCandidatePaths(command),
     Platform.isWindows,
     (path, isWindows) => isExecutableSync(path, isWindows, FileStat.statSync),
     orElse);