do not report _ as unused variable

pkg/analyzer/test/generated/resolver_test.dart

Index: pkg/analyzer/test/generated/resolver_test.dart
diff --git a/pkg/analyzer/test/generated/resolver_test.dart b/pkg/analyzer/test/generated/resolver_test.dart
index 4f24b3abe76efdf10461c06f6cef44bfdcb0349f..7ba6e181e6ac64c29e865b4904f67c8358d014cd 100644
--- a/pkg/analyzer/test/generated/resolver_test.dart
+++ b/pkg/analyzer/test/generated/resolver_test.dart
@@ -4220,6 +4220,21 @@ print(x) {}''');
     verify([source]);
   }
 
+  void test_unusedLocalVariable_inFor_underscore_ignored() {
+    enableUnusedLocalVariable = true;
+    Source source = addSource(r'''
+main() {
+  for (var _ in [1,2,3]) {
+    for (var __ in [4,5,6]) {
+      // do something
+    }
+  }
+}''');
+    resolve(source);
+    assertErrors(source);
+    verify([source]);
+  }
+
   void test_unusedLocalVariable_inFunction() {
     enableUnusedLocalVariable = true;
     Source source = addSource(r'''