do not report _ as unused variable

pkg/analyzer/lib/src/generated/resolver.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;
 
 import "dart:math" as math;
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/utilities_collection.dart';
@@ skipping 15302 matching lines @@
       _reportErrorForElement(HintCode.UNUSED_ELEMENT, element, [
         element.kind.displayName,
         element.displayName
       ]);
     }
     super.visitFunctionElement(element);
   }
 
   @override
   visitLocalVariableElement(LocalVariableElement element) {
-    if (!_isUsedElement(element)) {
+    if (!_isUsedElement(element) && !_isNamedUnderscore(element)) {
       HintCode errorCode;
       if (_usedElements.isCatchException(element)) {
         errorCode = HintCode.UNUSED_CATCH_CLAUSE;
       } else if (_usedElements.isCatchStackTrace(element)) {
         errorCode = HintCode.UNUSED_CATCH_STACK;
       } else {
         errorCode = HintCode.UNUSED_LOCAL_VARIABLE;
       }
       _reportErrorForElement(errorCode, element, [element.displayName]);
     }
@@ skipping 14 matching lines @@
   visitPropertyAccessorElement(PropertyAccessorElement element) {
     if (!_isUsedMember(element)) {
       _reportErrorForElement(HintCode.UNUSED_ELEMENT, element, [
         element.kind.displayName,
         element.displayName
       ]);
     }
     super.visitPropertyAccessorElement(element);
   }
 
+  bool _isNamedUnderscore(LocalVariableElement element) {
+    String name = element.name;
+    if (name != null) {
+      for (int index = name.length - 1; index >= 0; --index) {
+        if (name.codeUnitAt(index) != 0x5F) { // 0x5F => '_'
+          return false;
+        }
+      }
+      return true;
+    }
+    return false;
+  }
+
   bool _isReadMember(Element element) {
     if (element.isPublic) {
       return true;
     }
     if (element.isSynthetic) {
       return true;
     }
     return _usedElements.readMembers.contains(element.displayName);
   }
 
@@ skipping 79 matching lines @@
   }
 
   bool isCatchException(LocalVariableElement element) {
     return catchExceptionElements.contains(element);
   }
 
   bool isCatchStackTrace(LocalVariableElement element) {
     return catchStackTraceElements.contains(element);
   }
 }