[analysis_server] Verify with/without documentChanges for EditArguments tests

The tests by default run with documentChanges (set in `setUp`), but it's possible LSP-over-Legacy clients won't support it.

Change-Id: I40c65f05127c88e15a43f16669f64675285955dc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/406002
Reviewed-by: Samuel Rawlins <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: DanTup

pkg/analysis_server/test/shared/shared_edit_argument_tests.dart

@@ -28,6 +28,7 @@ mixin SharedEditArgumentTests
   Future<void> setUp() async {
     await super.setUp();
     setApplyEditSupport();
+    setDocumentChangesSupport();
   }
 
   test_comma_addArg_addsIfExists() async {
@@ -66,6 +67,30 @@ mixin SharedEditArgumentTests
     );
   }
 
+  test_documentChanges_supported() async {
+    // Ensure documentChanges are supported. The verification in
+    // LspChangeVerifier will verify the resulting edits match the capabilities.
+    setDocumentChangesSupport();
+    await _expectSimpleArgumentEdit(
+      params: '({ int? x })',
+      originalArgs: '(x: 1)',
+      edit: ArgumentEdit(name: 'x', newValue: 2),
+      expectedArgs: '(x: 2)',
+    );
+  }
+
+  test_documentChanges_unsupported() async {
+    // documentChanges are NOT supported. The verification in
+    // LspChangeVerifier will verify the resulting edits match the capabilities.
+    setDocumentChangesSupport(false);
+    await _expectSimpleArgumentEdit(
+      params: '({ int? x })',
+      originalArgs: '(x: 1)',
+      edit: ArgumentEdit(name: 'x', newValue: 2),
+      expectedArgs: '(x: 2)',
+    );
+  }
+
   test_format_multiline_insert_between() async {
     await _expectSimpleArgumentEdit(
       params: '({ int? x, int? y, int? children })',