Fix computation of errors to not run extra tasks

pkg/analyzer/lib/src/plugin/engine_plugin.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.plugin.engine_plugin;
 
 import 'package:analyzer/plugin/task.dart';
 import 'package:analyzer/src/generated/engine.dart'
     show InternalAnalysisContext;
 import 'package:analyzer/src/generated/error.dart' show AnalysisError;
@@ skipping 141 matching lines @@
   void registerExtensions(RegisterExtension registerExtension) {
     _registerTaskExtensions(registerExtension);
     _registerWorkManagerFactoryExtensions(registerExtension);
     _registerDartErrorsForSource(registerExtension);
     _registerDartErrorsForUnit(registerExtension);
     _registerHtmlErrors(registerExtension);
   }
 
   void _registerDartErrorsForSource(RegisterExtension registerExtension) {
     String id = DART_ERRORS_FOR_SOURCE_EXTENSION_POINT_ID;
-    registerExtension(id, BUILD_DIRECTIVES_ERRORS);
-    registerExtension(id, BUILD_LIBRARY_ERRORS);
     registerExtension(id, PARSE_ERRORS);
     registerExtension(id, SCAN_ERRORS);
   }
 
   void _registerDartErrorsForUnit(RegisterExtension registerExtension) {
     String id = DART_ERRORS_FOR_UNIT_EXTENSION_POINT_ID;
     registerExtension(id, LIBRARY_UNIT_ERRORS);
   }
 
   void _registerHtmlErrors(RegisterExtension registerExtension) {
@@ skipping 88 matching lines @@
    * a [WorkManagerFactory].
    */
   void _validateWorkManagerFactoryExtension(Object extension) {
     if (extension is! WorkManagerFactory) {
       String id = taskExtensionPoint.uniqueIdentifier;
       throw new ExtensionError(
           'Extensions to $id must be a WorkManagerFactory');
     }
   }
 }