Report HintCode.UNUSED_FIELD for fields which are referenced in constructor field initializers.

pkg/analyzer/lib/src/dart/ast/ast.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 analyzer.src.dart.ast.ast;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 9025 matching lines @@
     if (parent is Label) {
       return false;
     }
     // analyze usage
     if (parent is AssignmentExpression) {
       if (identical(parent.leftHandSide, target) &&
           parent.operator.type == TokenType.EQ) {
         return false;
       }
     }
+    if (parent is ConstructorFieldInitializer &&
+        identical(parent.fieldName, target)) {
+      return false;
+    }
     if (parent is ForEachStatement) {
       if (identical(parent.identifier, target)) {
         return false;
       }
     }
     return true;
   }
 
   @override
   bool inSetterContext() {
@@ skipping 1983 matching lines @@
 
   @override
   dynamic/*=E*/ accept/*<E>*/(AstVisitor/*<E>*/ visitor) =>
       visitor.visitYieldStatement(this);
 
   @override
   void visitChildren(AstVisitor visitor) {
     _expression?.accept(visitor);
   }
 }