Avoid NPE in package resolver if there's a problem running 'pub list'

pkg/analyzer/lib/src/analyzer_impl.dart

 // Copyright (c) 2013, 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_impl;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:io';
 
 import 'package:analyzer/file_system/physical_file_system.dart';
 import 'package:analyzer/source/package_map_provider.dart';
 import 'package:analyzer/source/package_map_resolver.dart';
 import 'package:analyzer/source/pub_package_map_provider.dart';
 import 'package:analyzer/src/error_formatter.dart';
 import 'package:analyzer/src/generated/java_core.dart' show JavaSystem;
 import 'package:analyzer/src/generated/java_engine.dart';
 
 import '../options.dart';
 import 'generated/constant.dart';
 import 'generated/element.dart';
 import 'generated/engine.dart';
 import 'generated/error.dart';
 import 'generated/java_io.dart';
 import 'generated/sdk_io.dart';
 import 'generated/source_io.dart';
+import 'package:analyzer/file_system/file_system.dart';
 
 DirectoryBasedDartSdk sdk;
 
 /**
  * The maximum number of sources for which AST structures should be kept in the cache.
  */
 const int _MAX_CACHE_SIZE = 512;
 
 /// Analyzes single library [File].
 class AnalyzerImpl {
@@ skipping 107 matching lines @@
     {
       JavaFile packageDirectory;
       if (options.packageRootPath != null) {
         packageDirectory = new JavaFile(options.packageRootPath);
         resolvers.add(new PackageUriResolver([packageDirectory]));
       } else {
         PubPackageMapProvider pubPackageMapProvider =
             new PubPackageMapProvider(PhysicalResourceProvider.INSTANCE, sdk);
         PackageMapInfo packageMapInfo = pubPackageMapProvider.computePackageMap(
             PhysicalResourceProvider.INSTANCE.getResource('.'));
-        resolvers.add(
-            new PackageMapUriResolver(
-                PhysicalResourceProvider.INSTANCE,
-                packageMapInfo.packageMap));
+        Map<String, List<Folder>> packageMap = packageMapInfo.packageMap;
+        if (packageMap != null) {
+          resolvers.add(
+              new PackageMapUriResolver(
+                  PhysicalResourceProvider.INSTANCE,
+                  packageMap));
+        }
       }
     }
     sourceFactory = new SourceFactory(resolvers);
     context = AnalysisEngine.instance.createAnalysisContext();
     context.sourceFactory = sourceFactory;
     Map<String, String> definedVariables = options.definedVariables;
     if (!definedVariables.isEmpty) {
       DeclaredVariables declaredVariables = context.declaredVariables;
       definedVariables.forEach((String variableName, String value) {
         declaredVariables.define(variableName, value);
@@ skipping 275 matching lines @@
     }
   }
 
   @override
   void logInformation2(String message, Object exception) {
     if (log) {
       stdout.writeln(message);
     }
   }
 }