Hook for overriding the ContextManager and some code clean-up (with more to follow)

pkg/analysis_server/test/socket_server_test.dart

Index: pkg/analysis_server/test/socket_server_test.dart
diff --git a/pkg/analysis_server/test/socket_server_test.dart b/pkg/analysis_server/test/socket_server_test.dart
index 8e74cefd4aed442dd12a1186ab063d930a9c1cae..ba79457b91a511457018a03dfb3a2f87d0920ac9 100644
--- a/pkg/analysis_server/test/socket_server_test.dart
+++ b/pkg/analysis_server/test/socket_server_test.dart
@@ -111,7 +111,7 @@ class SocketServerTest {
     manager.processPlugins([serverPlugin]);
     return new SocketServer(new AnalysisServerOptions(),
         DirectoryBasedDartSdk.defaultSdk, InstrumentationService.NULL_SERVICE,
-        serverPlugin, null);
+        serverPlugin, null, null);
   }
 }