[analysis_server] Enable "EditArgument" request to work over the legacy protocol

+ move all related tests to a shared mixin so they run for both servers.

Change-Id: I853e8f24948c07fdde2a5d6a64ddc65962357b6d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/404841
Commit-Queue: Brian Wilkerson <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Phil Quitslund <[email protected]>

Author: DanTup

pkg/analysis_server/lib/src/analysis_server.dart

@@ -1033,6 +1033,11 @@ abstract class AnalysisServer {
   /// 'lsp.notification' notification.
   void sendLspNotification(lsp.NotificationMessage notification);
 
+  /// Sends an LSP request with the given [params] to the client and waits for a
+  /// response. Completes with the raw [lsp.ResponseMessage] which could be an
+  /// error response.
+  Future<lsp.ResponseMessage> sendLspRequest(lsp.Method method, Object params);
+
   /// Sends an error notification to the user.
   void sendServerErrorNotification(
     String message,

pkg/analysis_server/lib/src/legacy_analysis_server.dart

@@ -719,6 +719,7 @@ class LegacyAnalysisServer extends AnalysisServer {
 
   /// Sends an LSP request to the server (wrapped in 'lsp.handle') and unwraps
   /// the LSP response from the result of the legacy response.
+  @override
   Future<lsp.ResponseMessage> sendLspRequest(
     lsp.Method method,
     Object params,

pkg/analysis_server/lib/src/lsp/handlers/custom/editable_arguments/handler_edit_argument.dart

@@ -10,7 +10,6 @@ import 'package:analysis_server/src/lsp/constants.dart';
 import 'package:analysis_server/src/lsp/error_or.dart';
 import 'package:analysis_server/src/lsp/handlers/custom/editable_arguments/editable_arguments_mixin.dart';
 import 'package:analysis_server/src/lsp/handlers/handlers.dart';
-import 'package:analysis_server/src/lsp/lsp_analysis_server.dart';
 import 'package:analysis_server/src/lsp/mapping.dart';
 import 'package:analysis_server/src/lsp/source_edits.dart';
 import 'package:analyzer/dart/analysis/results.dart';
@@ -286,14 +285,6 @@ class EditArgumentHandler extends SharedMessageHandler<EditArgumentParams, Null>
   /// Sends [workspaceEdit] to the client and returns `null` if applied
   /// successfully or an error otherwise.
   Future<ErrorOr<Null>> _sendEditToClient(WorkspaceEdit workspaceEdit) async {
-    var server = this.server;
-    if (server is! LspAnalysisServer) {
-      return error(
-        ErrorCodes.RequestFailed,
-        'Sending edits is currently only supported for clients using LSP directly',
-      );
-    }
-
     var editDescription = 'Edit argument';
     var editResponse = await server.sendLspRequest(
       Method.workspace_applyEdit,

pkg/analysis_server/lib/src/lsp/lsp_analysis_server.dart

@@ -861,9 +861,7 @@ class LspAnalysisServer extends AnalysisServer {
     channel.sendNotification(notification);
   }
 
-  /// Sends a request with the given [params] to the client and wait for a
-  /// response. Completes with the raw [ResponseMessage] which could be an
-  /// error response.
+  @override
   Future<ResponseMessage> sendLspRequest(Method method, Object params) {
     var requestId = nextRequestId++;
     var completer = Completer<ResponseMessage>();