Mark HintCode.CAN_BE_NULL_AFTER_NULL_AWARE as fixable.

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 'package:analysis_server/plugin/edit/fix/fix_core.dart';
 import 'package:analysis_server/plugin/protocol/protocol.dart'
     hide AnalysisError;
 import 'package:analysis_server/src/services/correction/fix.dart';
@@ skipping 3280 matching lines @@
 
   void test_removeUnnecessaryCast_assignment() {
     resolveTestUnit('''
 main(Object p) {
   if (p is String) {
     String v = ((p as String));
   }
 }
 ''');
     assertHasFix(
-        DartFixKind.REMOVE_UNNECASSARY_CAST,
+        DartFixKind.REMOVE_UNNECESSARY_CAST,
         '''
 main(Object p) {
   if (p is String) {
     String v = p;
   }
 }
 ''');
   }
 
   void test_removeUnusedCatchClause() {
@@ skipping 1301 matching lines @@
       int offset = resultCode.indexOf(search);
       positions.add(new Position(testFile, offset));
     }
     return positions;
   }
 
   void _performAnalysis() {
     while (context.performAnalysisTask().hasMoreWork);
   }
 }