Revert server test extension manager "fix" [TBR].

pkg/analysis_server/test/analysis_server_test.dart

Index: pkg/analysis_server/test/analysis_server_test.dart
diff --git a/pkg/analysis_server/test/analysis_server_test.dart b/pkg/analysis_server/test/analysis_server_test.dart
index 05fedf445375a97c696d0ae1237d83bd375b53c7..9f75123e0e4e0dc4f0285bfc7c0971e990a72761 100644
--- a/pkg/analysis_server/test/analysis_server_test.dart
+++ b/pkg/analysis_server/test/analysis_server_test.dart
@@ -123,14 +123,9 @@ import "../foo/foo.dart";
     channel = new MockServerChannel();
     resourceProvider = new MemoryResourceProvider();
     packageMapProvider = new MockPackageMapProvider();
-
+    ExtensionManager manager = new ExtensionManager();
     ServerPlugin serverPlugin = new ServerPlugin();
-
-    // Defer to the extension manager in AE for plugin registration.
-    AnalysisEngine.instance.userDefinedPlugins = [serverPlugin];
-    // Force registration.
-    AnalysisEngine.instance.taskManager;
-
+    manager.processPlugins([serverPlugin]);
     server = new AnalysisServer(
         channel,
         resourceProvider,