homebrew: use exec to run shimmed command

tools/apps/update_homebrew/bin/update_homebrew.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library update_homebrew;
 
 import 'dart:io';
 import 'dart:convert';
 import 'dart:async';
 import 'package:http/http.dart' as http;
@@ skipping 122 matching lines @@
 
   resource 'content_shell' do
     url '$urlBase/stable/release/${revisions['stable']}/$contentShellFile'
     version '$stableVersion'
     sha256 '${hashes['stable'][contentShellFile]}'
   end
 
   def shim_script target
     <<-EOS.undent
       #!/bin/bash
-      "#{prefix}/#{target}" "\$@"
+      exec "#{prefix}/#{target}" "\$@"
     EOS
   end
 
   def install
     dartium_binary = 'Chromium.app/Contents/MacOS/Chromium'
     prefix.install Dir['*']
     (bin+"dartium").write shim_script dartium_binary
 
     content_shell_binary = 'Content Shell.app/Contents/MacOS/Content Shell'
     prefix.install resource('content_shell')
@@ skipping 104 matching lines @@
         repository = tempDir.path;
       })
       .then((_) => runGit(
           ['clone', 'git@github.com:dart-lang/homebrew-dart.git', '.']))
       .then((_) => writeHomebrewInfo(channel, revision))
       .then((_) => runGit(['commit', '-a', '-m',
                            'Updated $channel branch to revision $revision']))
       .then((_) => runGit(['push']))
       .whenComplete(() => new Directory(repository).delete(recursive: true));
 }