Add @override when implement missing overrides for fields.

pkg/analysis_server/test/services/correction/fix_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 test.services.correction.fix;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/edit/fix/fix_core.dart';
 import 'package:analysis_server/plugin/edit/fix/fix_dart.dart';
@@ skipping 2126 matching lines @@
 }
 ''');
     await assertHasFix(
         DartFixKind.CREATE_MISSING_OVERRIDES,
         '''
 class A {
   var f;
 }
 
 class B implements A {
+  @override
   var f;
 }
 ''');
   }
 
   test_createMissingOverrides_functionTypeAlias() async {
     resolveTestUnit('''
 typedef int Binary(int left, int right);
 
 abstract class Emulator {
@@ skipping 172 matching lines @@
     await assertHasFix(
         DartFixKind.CREATE_MISSING_OVERRIDES,
         '''
 class A {
   int ma;
   void mb() {}
   double mc;
 }
 
 class B implements A {
+  @override
   int ma;
 
+  @override
   double mc;
 
   @override
   void mb() {
     // TODO: implement mb
   }
 }
 ''');
   }
 
@@ skipping 2657 matching lines @@
       int offset = resultCode.indexOf(search);
       positions.add(new Position(testFile, offset));
     }
     return positions;
   }
 
   void _performAnalysis() {
     while (context.performAnalysisTask().hasMoreWork);
   }
 }