Stop reporting diagnostics on synthetic identifiers (issue 38448)

Change-Id: Ief9b77a48526454db430bc131d3c1dbd04234dc1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/117761
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: bwilkerson

pkg/analyzer/lib/src/error/duplicate_definition_verifier.dart

@@ -343,6 +343,9 @@ class DuplicateDefinitionVerifier {
   void _checkDuplicateIdentifier(
       Map<String, Element> getterScope, SimpleIdentifier identifier,
       {Element element, Map<String, Element> setterScope}) {
+    if (identifier.isSynthetic) {
+      return;
+    }
     element ??= identifier.staticElement;
 
     // Fields define getters and setters, so check them separately.

pkg/analyzer/test/src/diagnostics/duplicate_definition_test.dart

@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analyzer/dart/analysis/features.dart';
+import 'package:analyzer/src/dart/error/syntactic_errors.dart';
 import 'package:analyzer/src/error/codes.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
@@ -316,6 +317,15 @@ class C {
       error(CompileTimeErrorCode.DUPLICATE_DEFINITION, 56, 3),
     ]);
   }
+
+  test_topLevel_syntheticParameters() async {
+    await assertErrorsInCode(r'''
+f(,[]) {}
+''', [
+      error(ParserErrorCode.MISSING_IDENTIFIER, 2, 1),
+      error(ParserErrorCode.MISSING_IDENTIFIER, 4, 1),
+    ]);
+  }
 }
 
 @reflectiveTest