Add code to locate a plugin within a package

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.
+ */
+class PluginLocator {
+  /**
+   * The key used in the `pubspec.yaml` file to specify the location of the
+   * analysis plugin.
+   */
+  static const String analysisPluginKey = 'analysis_plugin';
+
+  /**
+   * The name of the default plugin directory, located within the `tools`
+   * directory.
+   */
+  static const String defaultPluginFolderName = 'analysis_plugin';
+
+  /**
+   * The name of the `pubspec.yaml` file.
+   */
+  static const String pubspecFileName = 'pubspec.yaml';
+
+  /**
+   * The name of the default plugin directory, located within the `tools`
+   * directory.

[scheglov, 2017/02/05 19:48:07]

Update it?

[Brian Wilkerson, 2017/02/05 20:00:22]

Done

+   */
+  static const String toolsFolderName = 'tools';
+
+  /**
+   * The resource provider used to access the file system.
+   */
+  final ResourceProvider resourceProvider;
+
+  /**
+   * 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) {
+    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];
+          Folder pluginFolder =
+              packageFolder.getChildAssumingFolder(pluginPath);
+          if (pluginFolder.exists) {
+            return pluginFolder.path;
+          }
+        }
+      } catch (exception) {
+        // If we can't read the file, or if it isn't valid YAML, then ignore it.
+      }
+    }
+    Folder pluginFolder = packageFolder
+        .getChildAssumingFolder(toolsFolderName)
+        .getChildAssumingFolder(defaultPluginFolderName);
+    if (pluginFolder.exists) {
+      return pluginFolder.path;
+    }
+    return null;
+  }
+}