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

pkg/analysis_server/test/analysis/get_errors_test.dart

Index: pkg/analysis_server/test/analysis/get_errors_test.dart
diff --git a/pkg/analysis_server/test/analysis/get_errors_test.dart b/pkg/analysis_server/test/analysis/get_errors_test.dart
index 846990de003e1b378214636eaaa792e0c3bbf3ad..14203c97ca22e6376c2e78b85827cbd2a10fc62d 100644
--- a/pkg/analysis_server/test/analysis/get_errors_test.dart
+++ b/pkg/analysis_server/test/analysis/get_errors_test.dart
@@ -99,7 +99,7 @@ main() {
     // remove context, causes sending an "invalid file" error
     {
       Folder projectFolder = resourceProvider.getResource(projectPath);
-      server.contextDirectoryManager.removeContext(projectFolder);
+      server.contextManager.removeContext(projectFolder);
     }
     // wait for an error response
     return serverChannel.waitForResponse(request).then((Response response) {