Don't run 'pub list-package-dirs' if there is no pubpsec.lock file.

pkg/analysis_server/test/source/caching_put_package_map_provider_test.dart

 // Copyright (c) 2015, 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 test.source.caching_pub_package_map_provider;
 
 import 'dart:convert';
 import 'dart:core' hide Resource;
 import 'dart:io' as io;
 
@@ skipping 31 matching lines @@
 
     Folder newProj(Map result) {
       Map packages = result['packages'];
       packages.forEach((String name, String path) {
         resProvider.newFolder(path);
       });
       List<String> inputFiles = result['input_files'];
       for (String path in inputFiles) {
         resProvider.newFile(path, '');
       }
-      return resProvider.getResource(inputFiles[0]).parent;
+      Folder projectFolder = resProvider.getResource(inputFiles[0]).parent;
+      resProvider.newFile(projectFolder.path + '/pubspec.lock', '');
+      return projectFolder;
     }
 
     int mockWriteFile(File cacheFile, String content) {
       if (writeFileException) {
         throw 'simulated write failure: $cacheFile';
       }
       if (!cacheFile.exists) {
         resProvider.newFolder(cacheFile.parent.path);
         resProvider.newFile(cacheFile.path, content);
       } else {
@@ skipping 328 matching lines @@
       provider.newFolder(path);
     }
     int fileCount = _filePaths.length;
     for (int fileIndex = 0; fileIndex < fileCount; ++fileIndex) {
       String path = _filePaths[fileIndex];
       TimestampedData content = _fileContents[fileIndex];
       provider.newFile(path, content.data, content.modificationTime);
     }
   }
 }