Remove the old task model

pkg/analyzer/lib/src/task/dart_work_manager.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.
 
 library analyzer.src.task.dart_work_manager;
 
 import 'dart:collection';
 
 import 'package:analyzer/src/context/cache.dart';
 import 'package:analyzer/src/generated/engine.dart'
@@ skipping 456 matching lines @@
     bool needErrors = _shouldErrorsBeComputed(librarySource);
     if (needErrors) {
       addPriorityResult(librarySource, LIBRARY_ERRORS_READY);
     } else {
       var target = new LibrarySpecificUnit(librarySource, librarySource);
       addPriorityResult(target, RESOLVED_UNIT);
     }
   }
 
   bool _shouldErrorsBeComputed(Source source) =>
-      context.shouldErrorsBeAnalyzed(source, null);
+      context.shouldErrorsBeAnalyzed(source);
 
   static bool _isDartSource(AnalysisTarget target) {
     return target is Source && AnalysisEngine.isDartFileName(target.fullName);
   }
 }