null check for pub result in PubPackageMapProvider

pkg/analyzer/lib/source/pub_package_map_provider.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 source.pub_package_map_provider;
 
 import 'dart:collection';
 import 'dart:convert';
 import 'dart:io' as io;
 
@@ skipping 38 matching lines @@
     try {
       result = io.Process.runSync(
           executable,
           [PUB_LIST_COMMAND],
           workingDirectory: folder.path);
     } on io.ProcessException catch (exception, stackTrace) {
       AnalysisEngine.instance.logger.logInformation(
           "Error running pub $PUB_LIST_COMMAND\n$exception\n$stackTrace");
     }
     if (result == null || result.exitCode != 0) {
+      String exitCode =
+          result != null ? 'exit code ${result.exitCode}' : 'null';
       AnalysisEngine.instance.logger.logInformation(
-          "pub $PUB_LIST_COMMAND failed: exit code ${result.exitCode}");
+          "pub $PUB_LIST_COMMAND failed: $exitCode");
       return _error(folder);
     }
     try {
       return parsePackageMap(result.stdout, folder);
     } catch (exception, stackTrace) {
       AnalysisEngine.instance.logger.logError(
           "Malformed output from pub $PUB_LIST_COMMAND\n$exception\n$stackTrace");
     }
 
     return _error(folder);
@@ skipping 56 matching lines @@
    */
   PackageMapInfo _error(Folder folder) {
     // Even if an error occurs, we still need to know the dependencies, so that
     // we'll know when to try running "pub list-package-dirs" again.
     // Unfortunately, "pub list-package-dirs" doesn't tell us dependencies when
     // an error occurs, so just assume there is one dependency, "pubspec.lock".
     List<String> dependencies = <String>[join(folder.path, PUBSPEC_LOCK_NAME)];
     return new PackageMapInfo(null, dependencies.toSet());
   }
 }