do not report _ as unused variable

pkg/analyzer/test/generated/resolver_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library engine.resolver_test;
 
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
@@ skipping 4202 matching lines @@
   } catch (exception, stackTrace) {
     print('exception at $stackTrace');
   }
 }
 print(x) {}''');
     resolve(source);
     assertErrors(source);
     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'''
 main() {
   var v = 1;
   v = 2;
 }''');
     resolve(source);
     assertErrors(source, [HintCode.UNUSED_LOCAL_VARIABLE]);
     verify([source]);
@@ skipping 9519 matching lines @@
         // check propagated type
         FunctionType propagatedType = node.propagatedType as FunctionType;
         expect(propagatedType.returnType, test.typeProvider.stringType);
       } on AnalysisException catch (e, stackTrace) {
         thrownException[0] = new CaughtException(e, stackTrace);
       }
     }
     return null;
   }
 }