analyzer: track used calls to 'call' on an extension

Fixes #55781

Change-Id: I34eb02d4c0f6a44afe98c0224bed779972a415e6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/387560
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

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

@@ -1145,6 +1145,8 @@ class UsedLocalElements {
   void addElement(Element? element) {
     if (element is JoinPatternVariableElementImpl) {
       elements.addAll(element.transitiveVariables);
+    } else if (element is ExecutableMember) {
+      elements.add(element.declaration);
     } else if (element != null) {
       elements.add(element);
     }

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

@@ -991,6 +991,18 @@ main() {
     ]);
   }
 
+  test_method_isUsed_call_inExtension() async {
+    await assertNoErrorsInCode(r'''
+extension<T> on T {
+  void call() {}
+}
+
+void f() {
+  (<T>(T t) => t())(7);
+}
+''');
+  }
+
   test_method_isUsed_hasPragma_vmEntryPoint() async {
     pragma;
     await assertNoErrorsInCode(r'''
@@ -1366,6 +1378,16 @@ void main() {
 ''');
   }
 
+  test_method_notUsed_call_inExtension() async {
+    await assertErrorsInCode(r'''
+extension<T> on T {
+  void call() {}
+}
+''', [
+      error(WarningCode.UNUSED_ELEMENT, 27, 4),
+    ]);
+  }
+
   test_method_notUsed_hasSameNameAsUsed() async {
     await assertErrorsInCode(r'''
 class A {