[analysis_server] Fix missing trailing comma when inserting arguments before solo child/children

The logic here was bad, assuming we didn't need a trailing comma when inserting an arg before a solo argument (which can happen for child/children).

Fixes flutter/devtools#8921

Change-Id: I38f9003cdc8b499d6417cac7989c65a4760416a6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/411140
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Elliott Brooks <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: DanTup

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

@@ -431,15 +431,16 @@ class EditArgumentHandler extends SharedMessageHandler<EditArgumentParams, Null>
       (false, false) => '',
     };
 
-    // The suffix depends on whether there is an argument after us and whether
-    // we are multiline. If there is an argument after us, there is already
-    // the correct whitespace and comma after our insertion point.
+    // The suffix depends on whether there is an argument after us, whether
+    // we are multiline and whether there is already a comma after us.
     var codeSuffix = switch ((
       isMultiline,
       hasArgumentsAfter,
       commaFollowsInsertion,
     )) {
-      (_, true, _) => '',
+      (_, true, true) => '',
+      (true, true, false) => ',',
+      (false, true, false) => ', ',
       (true, false, false) => ',',
       (true, false, true) => '',
       (false, false, _) => '',

pkg/analysis_server/test/shared/shared_edit_argument_tests.dart

@@ -158,6 +158,31 @@ mixin SharedEditArgumentTests
     );
   }
 
+  test_named_addAfterNamed_beforeChild_noOthers() async {
+    await _expectSimpleArgumentEdit(
+      params: '({ int? y, Widget? child })',
+      originalArgs: '(child: null)',
+      edit: ArgumentEdit(name: 'y', newValue: 2),
+      expectedArgs: '(y: 2, child: null)',
+    );
+  }
+
+  test_named_addAfterNamed_beforeChild_noOthers_multiline() async {
+    await _expectSimpleArgumentEdit(
+      params: '({ int? y, Widget? child })',
+      originalArgs: '''\n
+      (
+        child: null
+      )''',
+      edit: ArgumentEdit(name: 'y', newValue: 2),
+      expectedArgs: '''\n
+      (
+        y: 2,
+        child: null
+      )''',
+    );
+  }
+
   test_named_addAfterNamed_beforeChildAtEnd() async {
     await _expectSimpleArgumentEdit(
       params: '({ int? x, int? y, Widget? child })',
@@ -167,6 +192,31 @@ mixin SharedEditArgumentTests
     );
   }
 
+  test_named_addAfterNamed_beforeChildren_noOthers() async {
+    await _expectSimpleArgumentEdit(
+      params: '({ int? y, List<Widget>? children })',
+      originalArgs: '(children: [])',
+      edit: ArgumentEdit(name: 'y', newValue: 2),
+      expectedArgs: '(y: 2, children: [])',
+    );
+  }
+
+  test_named_addAfterNamed_beforeChildren_noOthers_multiline() async {
+    await _expectSimpleArgumentEdit(
+      params: '({ int? y, List<Widget>? children })',
+      originalArgs: '''\n
+      (
+        children: []
+      )''',
+      edit: ArgumentEdit(name: 'y', newValue: 2),
+      expectedArgs: '''\n
+      (
+        y: 2,
+        children: []
+      )''',
+    );
+  }
+
   test_named_addAfterNamed_beforeChildrenAtEnd() async {
     await _expectSimpleArgumentEdit(
       params: '({ int? x, int? y, List<Widget>? children })',