Improve performance of PluginLocator by caching results

pkg/analysis_server/lib/src/plugin/plugin_locator.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:analyzer/file_system/file_system.dart';
 import 'package:yaml/yaml.dart';
 
 /**
  * An object used to locate a plugin within a package.
  */
@@ skipping 19 matching lines @@
    * The name of the `tools` directory, in which the default plugin directory is
    * located.
    */
   static const String toolsFolderName = 'tools';
 
   /**
    * The resource provider used to access the file system.
    */
   final ResourceProvider resourceProvider;
 
+  final Map<String, String> pluginMap = <String, String>{};
+
   /**
    * Initialize a newly created plugin locator to use the given
    * [resourceProvider] to access the file system.
    */
   PluginLocator(this.resourceProvider);
 
   /**
    * Given the root directory of a package (the [packageRoot]), return the path
    * to the plugin associated with the package, or `null` if there is no plugin
    * associated with the package.
    *
    * This will look first in the `pubspec.yaml` file in the package root for a
    * key indicating where the plugin is located. If such a key is not defined,
    * then it will fall back to a well known location within the package.
    *
    * This method does not validate the content of the plugin directory before
    * returning it.
    */
   String findPlugin(String packageRoot) {
+    return pluginMap.putIfAbsent(packageRoot, () => _findPlugin(packageRoot));
+  }
+
+  /**
+   * The implementation of [findPlugin].
+   */
+  String _findPlugin(String packageRoot) {
     Folder packageFolder = resourceProvider.getFolder(packageRoot);
     File pubspecFile = packageFolder.getChildAssumingFile(pubspecFileName);
     if (pubspecFile.exists) {
       try {
         YamlDocument document = loadYamlDocument(pubspecFile.readAsStringSync(),
             sourceUrl: pubspecFile.toUri());
         YamlNode contents = document.contents;
         if (contents is YamlMap) {
           String pluginPath = contents[analysisPluginKey];
           if (pluginPath != null) {
@@ skipping 10 matching lines @@
     }
     Folder pluginFolder = packageFolder
         .getChildAssumingFolder(toolsFolderName)
         .getChildAssumingFolder(defaultPluginFolderName);
     if (pluginFolder.exists) {
       return pluginFolder.path;
     }
     return null;
   }
 }