[analysis_server] Don't insert trailing commas when editing named argument labels

Change-Id: I691b19eebd8d01796b5763c066573de52192a8a3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/234740
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart

@@ -74,7 +74,9 @@ class ArgListContributor extends DartCompletionContributor {
           // If there's a replacement length and the preference is to replace,
           // we should not include colons/commas.
           appendColon: appendColon && !willReplace,
-          appendComma: appendComma && !willReplace,
+          // Commas should always be suppressed when we're not inserting colons:
+          //     ke^: Key()
+          appendComma: appendComma && appendColon && !willReplace,
           replacementLength: replacementLength);
     }
   }

pkg/analysis_server/test/services/completion/dart/arglist_contributor_test.dart

@@ -168,6 +168,27 @@ build() => new Row(
     });
   }
 
+  Future<void> test_flutter_InstanceCreationExpression_3() async {
+    // Ensure a trailing comma is not added when only replacing the name.
+    writeTestPackageConfig(flutter: true);
+
+    addTestSource('''
+import 'package:flutter/material.dart';
+
+build() => new Row(
+    ke^: null,
+  );
+''');
+
+    var response = await computeSuggestions2();
+    _checkNamedArguments(response).containsMatch((suggestion) {
+      suggestion
+        ..completion.isEqualTo('key')
+        ..defaultArgumentListString.isNull
+        ..hasSelection(offset: 3);
+    });
+  }
+
   Future<void>
       test_flutter_InstanceCreationExpression_children_dynamic() async {
     // Ensure we don't generate unneeded <dynamic> param if a future API doesn't