fix #27088, future then upwards inference

pkg/analyzer/test/src/task/strong/strong_test_helper.dart

 // Copyright (c) 2015, 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.
 
 // TODO(jmesserly): this file needs to be refactored, it's a port from
 // package:dev_compiler's tests
 library analyzer.test.src.task.strong.strong_test_helper;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 83 matching lines @@
       var source = unit.source;
       if (source.uri.scheme == 'dart') continue;
 
       var librarySource = context.getLibrariesContaining(source).single;
       var resolved = context.resolveCompilationUnit2(source, librarySource);
 
       errors.addAll(context.computeErrors(source).where((e) =>
           // TODO(jmesserly): these are usually intentional dynamic calls.
           e.errorCode.name != 'UNDEFINED_METHOD' &&
           // We don't care about any of these:
-          e.errorCode != HintCode.UNNECESSARY_CAST &&
           e.errorCode != HintCode.UNUSED_ELEMENT &&
           e.errorCode != HintCode.UNUSED_FIELD &&
           e.errorCode != HintCode.UNUSED_IMPORT &&
           e.errorCode != HintCode.UNUSED_LOCAL_VARIABLE &&
           e.errorCode != TodoCode.TODO));
       _expectErrors(context, resolved, errors);
     }
   }
 
   return initialLibrary;
@@ skipping 284 matching lines @@
 
   @override
   Source resolveAbsolute(Uri uri, [Uri actualUri]) {
     if (uri.scheme == 'package') {
       return (provider.getResource('/packages/' + uri.path) as File)
           .createSource(uri);
     }
     return super.resolveAbsolute(uri, actualUri);
   }
 }