[analysis_server] Fix handling of null responses for LSP requests over DTD

The DTD client requires that the `type` field is populated in DTD responses, but this code previously would omit it when the result was `null` (because it used `result?.runtimeType`).

Change-Id: I5a6e0927e9c02f2abb690aefceca8f8b003cd43b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/394484
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Keerti Parthasarathy <[email protected]>
Commit-Queue: Keerti Parthasarathy <[email protected]>

Author: DanTup

pkg/analysis_server/lib/src/lsp/handlers/custom/handler_experimental_echo.dart

@@ -29,6 +29,14 @@ class ExperimentalEchoHandler extends SharedMessageHandler<Object?, Object?> {
     MessageInfo message,
     CancellationToken token,
   ) async {
+    // The DTD client automatically converts `null` params to an empty map, but
+    // (because of a previous bug) we want to test null results. So if the
+    // params are an empty map, return null. This is tested by
+    // `test_service_success_echo_nullResponse` in `SharedDtdTests`.
+    if (params is Map && params.isEmpty) {
+      return success(null);
+    }
+
     return success(params);
   }
 }

pkg/analysis_server/lib/src/services/dart_tooling_daemon/dtd_services.dart

@@ -110,7 +110,10 @@ class DtdServices {
       // boolean (for example). This means we need to put the result in a
       // field, which we're calling 'result'.
       (result) => completer.complete({
-        'type': result?.runtimeType.toString(),
+        // result can be null, but DTD requires that we have a `type`, so don't
+        // use `?.` here because it results in a missing `type` instead of
+        // `Null`.
+        'type': result.runtimeType.toString(),
         'result': result,
       }),
     );

pkg/analysis_server/test/shared/shared_dtd_tests.dart

@@ -301,11 +301,26 @@ mixin SharedDtdTests on LspRequestHelpersMixin {
       params: {'a': 'b'},
     );
 
-    var result = response.result['result'] as Map<String, Object?>;
+    var result = response.result['result'] as Map<String, Object?>?;
 
     expect(result, equals({'a': 'b'}));
   }
 
+  test_service_success_echo_nullResponse() async {
+    await initializeServer();
+    await sendConnectToDtdRequest(registerExperimentalHandlers: true);
+
+    var response = await dtd.call(
+      lspServiceName,
+      CustomMethods.experimentalEcho.toString(),
+    );
+
+    var result = response.result['result'] as Map<String, Object?>?;
+
+    expect(response.type, 'Null');
+    expect(result, isNull);
+  }
+
   @SkippedTest(reason: 'Shared LSP methods are currently disabled')
   test_service_success_hover() async {
     var code = TestCode.parse('''