Fix NPE when a resolver provider is given

pkg/analyzer_cli/lib/src/driver.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 analyzer_cli.src.driver;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
@@ skipping 505 matching lines @@
     } catch (_) {
       // Ignore and fall through to null.
     }
 
     return null;
   }
 
   _PackageInfo _findPackages(CommandLineOptions options) {
     if (packageResolverProvider != null) {
       // The resolver provider will do all the work later.
-      return null;
+      return new _PackageInfo(null, null);
     }
 
     Packages packages;
     Map<String, List<file_system.Folder>> packageMap;
 
     if (options.packageConfigPath != null) {
       String packageConfigPath = options.packageConfigPath;
       Uri fileUri = new Uri.file(packageConfigPath);
       try {
         File configFile = new File.fromUri(fileUri).absolute;
@@ skipping 295 matching lines @@
     for (var package in packages) {
       var packageName = path.basename(package.path);
       var realPath = package.resolveSymbolicLinksSync();
       result[packageName] = [
         PhysicalResourceProvider.INSTANCE.getFolder(realPath)
       ];
     }
     return result;
   }
 }