Roll Observatory packages and add a roll script

packages/which/lib/src/candidate_paths.dart

 
 library which.src.candidate_paths;
 
 import 'dart:io';
 
 import 'package:path/path.dart';
 
 Iterable<String> getCandidatePaths(String command, Map<String, String> environment, bool isWindows, Context context) {
   if (context.isAbsolute(command)) return [command];
 
@@ skipping 16 matching lines @@
 
   pathEnv.insert(0, context.current);
   var pathExt = splitEnvVar('PATHEXT', ".EXE");
   if (command.contains('.')) pathExt.insert(0, '');
   return noExtPaths.expand((commandPath) =>
       pathExt.map((pathExtEntry) => commandPath + pathExtEntry));
 }
 
 Iterable<String> getRealCandidatePaths(String command) =>
     getCandidatePaths(command, Platform.environment, Platform.isWindows, context);