Fix NPE in ContextManager

pkg/analysis_server/lib/src/context_manager.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 context.directory.manager;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/instrumentation/instrumentation.dart';
 import 'package:analyzer/source/package_map_provider.dart';
 import 'package:analyzer/source/package_map_resolver.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/java_io.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/generated/source_io.dart';
 import 'package:path/path.dart' as pathos;
 import 'package:watcher/watcher.dart';
+import 'package:analyzer/src/generated/java_engine.dart';
 
 /**
  * The name of `packages` folders.
  */
 const String PACKAGES_NAME = 'packages';
 
 /**
  * File name of pubspec files.
  */
 const String PUBSPEC_NAME = 'pubspec.yaml';
@@ skipping 419 matching lines @@
     _addSourceFiles(changeSet, folder, info);
     applyChangesToContext(folder, changeSet);
     return info;
   }
 
   /**
    * Clean up and destroy the context associated with the given folder.
    */
   void _destroyContext(Folder folder) {
     _contexts[folder].changeSubscription.cancel();
+    removeContext(folder);
     _contexts.remove(folder);
-    removeContext(folder);
   }
 
   /**
    * Extract a new [pubspecFile]-based context from [oldInfo].
    */
   void _extractContext(_ContextInfo oldInfo, File pubspecFile) {
     Folder newFolder = pubspecFile.parent;
     _ContextInfo newInfo = _createContext(newFolder, pubspecFile, []);
     newInfo.parent = oldInfo;
     // prepare sources to extract
@@ skipping 276 matching lines @@
     return excludes(resource.path);
   }
 
   /**
    * Returns `true` if [path] is the pubspec file of this context.
    */
   bool isPubspec(String path) {
     return path == pubspecPath;
   }
 }