Roll Observatory packages and add a roll script

packages/which/lib/src/is_executable.dart

 
 library which.src.is_executable;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:when/when.dart';
 
 import 'has_permission.dart';
 
@@ skipping 13 matching lines @@
   // There is no concept of executable on windows.
   if (isWindows) return true;
 
   // TODO: This currently produces false positives (returns true when it
   //       shouldn't) when the uid/gid of current user and executable don't
   //       match.  Fix if/when uid/gid support is added:
   //       http://dartbug.com/22037.
   return FilePermissionRole.values.any((role) =>
       hasPermission(stat.mode, FilePermission.EXECUTE, role: role));
 }