Issue 48682. Run 'dart pub' in PluginManager.

Bug: https://github.com/dart-lang/sdk/issues/48682
Change-Id: Ifc024b381bce23b914a6849c9d82cf89292706a3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/240901
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

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

@@ -632,13 +632,8 @@ class PluginManager {
         .getChildAssumingFolder(file_paths.dotDartTool)
         .getChildAssumingFile(file_paths.packageConfigJson);
     if (pubCommand != null) {
-      var vmPath = Platform.executable;
-      var pubPath = path.join(path.dirname(vmPath), 'pub');
-      if (Platform.isWindows) {
-        // Process.run requires the `.bat` suffix on Windows
-        pubPath = '$pubPath.bat';
-      }
-      var result = Process.runSync(pubPath, <String>[pubCommand],
+      var result = Process.runSync(
+          Platform.executable, <String>['pub', pubCommand],
           stderrEncoding: utf8,
           stdoutEncoding: utf8,
           workingDirectory: pluginFolder.path,