Clean-up the registration of plugins

pkg/analyzer_cli/lib/src/driver.dart

Index: pkg/analyzer_cli/lib/src/driver.dart
diff --git a/pkg/analyzer_cli/lib/src/driver.dart b/pkg/analyzer_cli/lib/src/driver.dart
index 752cb1e87597f572754ca4169c4936a35a1a6f7d..f69e935f5dd49b5fa33944eed2c51ab043a17303 100644
--- a/pkg/analyzer_cli/lib/src/driver.dart
+++ b/pkg/analyzer_cli/lib/src/driver.dart
@@ -35,6 +35,7 @@ import 'package:package_config/packages.dart' show Packages;
 import 'package:package_config/packages_file.dart' as pkgfile show parse;
 import 'package:package_config/src/packages_impl.dart' show MapPackages;
 import 'package:path/path.dart' as path;
+import 'package:plugin/manager.dart';
 import 'package:plugin/plugin.dart';
 import 'package:yaml/yaml.dart';
 
@@ -500,12 +501,14 @@ class Driver {
 
   void _processPlugins() {
     List<Plugin> plugins = <Plugin>[];
+    plugins.addAll(AnalysisEngine.instance.requiredPlugins);
+    plugins.add(AnalysisEngine.instance.commandLinePlugin);
+    plugins.add(AnalysisEngine.instance.optionsPlugin);
     plugins.add(linterPlugin);
     plugins.addAll(_userDefinedPlugins);
-    AnalysisEngine.instance.userDefinedPlugins = plugins;
 
-    // This ensures that AE extension manager processes plugins.
-    AnalysisEngine.instance.taskManager;
+    ExtensionManager manager = new ExtensionManager();
+    manager.processPlugins(plugins);
   }
 
   /// Analyze a single source.