Issue 53335. Extension types. Ignore external fields.

Bug: #53335
Change-Id: Idccb2bbd6b72c8293ab3a89ac0b327c21b94becd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/322982
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

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

@@ -257,9 +257,10 @@ class DuplicateDefinitionVerifier {
     if (declarationElement == null) return;
 
     final primaryConstructorName = element.constructors.first.name;
+    final representationGetter = element.representation.getter!;
     _getElementContext(declarationElement)
-        .constructorNames
-        .add(primaryConstructorName);
+      ..constructorNames.add(primaryConstructorName)
+      ..instanceGetters[representationGetter.name] = representationGetter;
 
     _checkClassMembers(element, node.members);
   }

pkg/analyzer/lib/src/generated/error_verifier.dart

@@ -2870,7 +2870,7 @@ class ErrorVerifier extends RecursiveAstVisitor<void>
       return;
     }
 
-    if (node.isStatic) {
+    if (node.isStatic || node.externalKeyword != null) {
       return;
     }
 

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

@@ -1222,6 +1222,36 @@ extension type E(int it) {
     ]);
   }
 
+  test_instance_representation_getter() async {
+    await assertErrorsInCode(r'''
+extension type E(int it) {
+  int get it => 0;
+}
+''', [
+      error(CompileTimeErrorCode.DUPLICATE_DEFINITION, 37, 2,
+          contextMessages: [message('/home/test/lib/test.dart', 21, 2)]),
+    ]);
+  }
+
+  test_instance_representation_method() async {
+    await assertErrorsInCode(r'''
+extension type E(int it) {
+  void it() {}
+}
+''', [
+      error(CompileTimeErrorCode.DUPLICATE_DEFINITION, 34, 2,
+          contextMessages: [message('/home/test/lib/test.dart', 21, 2)]),
+    ]);
+  }
+
+  test_instance_representation_setter() async {
+    await assertNoErrorsInCode(r'''
+extension type E(int it) {
+  set it(int _) {}
+}
+''');
+  }
+
   test_instance_setter_getter() async {
     await assertNoErrorsInCode(r'''
 extension type E(int it) {

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

@@ -25,6 +25,14 @@ extension type E(int it) {
     ]);
   }
 
+  test_instance_field_external() async {
+    await assertNoErrorsInCode('''
+extension type E(int it) {
+  external int foo;
+}
+''');
+  }
+
   test_instance_getter() async {
     await assertNoErrorsInCode('''
 extension type E(int it) {