fix #27363, support `@checked` on fields

pkg/analyzer/lib/src/dart/element/element.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.element.element;
 
 import 'dart:collection';
 import 'dart:math' show min;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 7180 matching lines @@
   ParameterElementImpl_ofImplicitSetter(
       PropertyAccessorElementImpl_ImplicitSetter setter)
       : setter = setter,
         super('_${setter.variable.name}', setter.variable.nameOffset) {
     enclosingElement = setter;
     synthetic = true;
     parameterKind = ParameterKind.REQUIRED;
   }
 
   @override
+  bool get isCovariant {
+    if (inheritsCovariant) {
+      return true;
+    }
+    for (ElementAnnotationImpl annotation in setter.variable.metadata) {
+      if (annotation.isCovariant) {
+        return true;
+      }
+    }
+    return false;
+  }
+
+  @override
   DartType get type => setter.variable.type;
 
   @override
   void set type(DartType type) {
     assert(false); // Should never be called.
   }
 }
 
 /**
  * A mixin that provides a common implementation for methods defined in
@@ skipping 1140 matching lines @@
 
   @override
   void visitElement(Element element) {
     int offset = element.nameOffset;
     if (offset != -1) {
       map[offset] = element;
     }
     super.visitElement(element);
   }
 }