Minor clean-up and bug fix

pkg/analysis_server/lib/src/plugin/plugin_manager.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:io' show Platform;
 
 import 'package:analysis_server/src/plugin/notification_manager.dart';
 import 'package:analyzer/context/context_root.dart' as analyzer;
@@ skipping 194 matching lines @@
       this.notificationManager, this.instrumentationService);
 
   /**
    * Add the plugin with the given [path] to the list of plugins that should be
    * used when analyzing code for the given [contextRoot]. If the plugin had not
    * yet been started, then it will be started by this method.
    */
   Future<Null> addPluginToContextRoot(
       analyzer.ContextRoot contextRoot, String path) async {
     PluginInfo plugin = _pluginMap[path];
-    bool isNew = false;
-    if (plugin == null) {
-      isNew = true;
+    bool isNew = plugin == null;
+    if (isNew) {
       List<String> pluginPaths = _pathsFor(path);
+      if (pluginPaths == null) {
+        return;
+      }
       plugin = new PluginInfo(path, pluginPaths[0], pluginPaths[1],
           notificationManager, instrumentationService);
       _pluginMap[path] = plugin;
       if (pluginPaths[0] != null) {
         PluginSession session = await plugin.start(byteStorePath);
         session.onDone.then((_) {
           _pluginMap.remove(path);
         });
       }
     }
@@ skipping 151 matching lines @@
       if (!pluginFile.exists) {
         return null;
       }
       File packagesFile = pluginFolder.getChildAssumingFile('.packages');
       if (!packagesFile.exists) {
         if (runPub) {
           // TODO(brianwilkerson) Run pub in the pluginFolder.
           if (!packagesFile.exists) {
             packagesFile = null;
           }
+        } else {
+          packagesFile = null;
         }
-        packagesFile = null;
       }
       return <String>[pluginFile.path, packagesFile?.path];
     }
 
     Folder pluginFolder = resourceProvider.getFolder(pluginPath);
     if (!needToCopy(pluginFolder)) {
       return computePaths(pluginFolder);
     }
     //
     // Copy the plugin directory to a unique subdirectory of the plugin
@@ skipping 198 matching lines @@
   Future<Null> stop() {
     if (channel == null) {
       throw new StateError('Cannot stop a plugin that is not running.');
     }
     // TODO(brianwilkerson) Ensure that the isolate is killed if it does not
     // terminate normally.
     sendRequest(new PluginShutdownParams());
     return pluginStoppedCompleter.future;
   }
 }