Update PluginWatcher to be more efficient

pkg/analysis_server/lib/src/plugin/plugin_watcher.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 'package:analysis_server/src/plugin/plugin_locator.dart';
 import 'package:analysis_server/src/plugin/plugin_manager.dart';
 import 'package:analyzer/context/context_root.dart';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/source/package_map_resolver.dart';
 import 'package:analyzer/src/dart/analysis/driver.dart';
@@ skipping 34 matching lines @@
   PluginWatcher(this.resourceProvider, this.manager)
       : _locator = new PluginLocator(resourceProvider);
 
   /**
    * The context manager has just added the given analysis [driver]. This method
    * must be called before the driver has been allowed to perform any analysis.
    */
   void addedDriver(AnalysisDriver driver, ContextRoot contextRoot) {
     _driverInfo[driver] = new _DriverInfo(
         contextRoot, <String>[contextRoot.root, _getSdkPath(driver)]);
-    driver.results.listen((AnalysisResult result) {
-      List<String> addedPluginPaths = _checkPluginsFor(driver);
+    driver.fsState.knownFilesSetChanges.listen((KnownFilesSetChange change) {
+      List<String> addedPluginPaths = _checkPluginsFor(driver, change);
       for (String pluginPath in addedPluginPaths) {
         manager.addPluginToContextRoot(contextRoot, pluginPath);
       }
     });
   }
 
   /**
    * The context manager has just removed the given analysis [driver].
    */
   void removedDriver(AnalysisDriver driver) {
     _DriverInfo info = _driverInfo[driver];
     if (info == null) {
       throw new StateError('Cannot remove a driver that was not added');
     }
     manager.removedContextRoot(info.contextRoot);
     _driverInfo.remove(driver);
   }
 
   /**
    * Check all of the files that have been analyzed so far by the given [driver]
    * to see whether any of them are in a package that had not previously been
    * seen that defines a plugin. Return a list of the roots of all such plugins
    * that are found.
    */
-  List<String> _checkPluginsFor(AnalysisDriver driver) {
+  List<String> _checkPluginsFor(
+      AnalysisDriver driver, KnownFilesSetChange change) {
+    _DriverInfo info = _driverInfo[driver];
+    if (info == null) {
+      // The driver must have been removed prior to getting the notification of
+      // newly analyzed files.
+      return const <String>[];
+    }
+    List<String> packageRoots = info.packageRoots;
+    FileSystemState fileSystemState = driver.fsState;
     AbsolutePathContext context = resourceProvider.absolutePathContext;
-    List<String> packageRoots = _driverInfo[driver].packageRoots;
 
     bool isInRoot(String path) {
       for (String root in packageRoots) {
         if (context.isWithin(root, path)) {
           return true;
         }
       }
       return false;
     }
 
     String getPackageRoot(String path, Uri uri) {
       List<String> segments = uri.pathSegments.toList();
       segments[0] = 'lib';
       String suffix = resourceProvider.pathContext.joinAll(segments);
       return path.substring(0, path.length - suffix.length - 1);
     }
 
     List<String> addedPluginPaths = <String>[];
-    for (FileState state in driver.fsState.knownFiles) {
-      String path = state.path;
+    for (String path in change.added) {
+      FileState state = fileSystemState.getFileForPath(path);
       if (!isInRoot(path)) {
         // Found a file not in a previously known package.
         Uri uri = state.uri;
         if (PackageMapUriResolver.isPackageUri(uri)) {
           String packageRoot = getPackageRoot(path, uri);
           packageRoots.add(packageRoot);
           String pluginPath = _locator.findPlugin(packageRoot);
           if (pluginPath != null) {
             addedPluginPaths.add(pluginPath);
           }
@@ skipping 30 matching lines @@
    * A list of the absolute paths of directories inside of which we have already
    * searched for a plugin.
    */
   final List<String> packageRoots;
 
   /**
    * Initialize a newly created information holder.
    */
   _DriverInfo(this.contextRoot, this.packageRoots);
 }