Add trailing commas after Flutter instance creation params fixes (flutter-intellij/#f978).

pkg/analysis_server/lib/src/services/correction/fix_internal.dart

Index: pkg/analysis_server/lib/src/services/correction/fix_internal.dart
diff --git a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
index 15119852ffc8b3714d1bc49716fa0d0ed1e35c9c..8064e8b45a5872706ddd88807322f501fc947555 100644
--- a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
+++ b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
@@ -640,7 +640,13 @@ class FixProcessor {
       ParameterElement element =
           parameters.firstWhere((p) => p.name == paramName, orElse: () => null);
       String defaultValue = getDefaultStringParameterValue(element);
-      sb.append('$paramName: $defaultValue'); // TODO(pq): add trailing comma
+      sb.append('$paramName: $defaultValue');
+
+      // Insert a trailing comma after Flutter instance creation params.
+      InstanceCreationExpression newExpr = identifyNewExpression(node);
+      if (newExpr != null && isFlutterInstanceCreationExpression(newExpr)) {
+        sb.append(',');
+      }
 
       _insertBuilder(sb, null);
       _addFix(DartFixKind.ADD_MISSING_REQUIRED_ARGUMENT, [paramName]);