Split BuildSourceClosuresTask into two tasks; remove unnecessary computation.

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/task/dart.dart';
 import 'package:analyzer/src/task/general.dart';
 import 'package:analyzer/src/task/html.dart';
@@ skipping 52 matching lines @@
     // Register Dart tasks.
     //
     registerExtension(taskId, BuildClassConstructorsTask.DESCRIPTOR);
     registerExtension(taskId, BuildCompilationUnitElementTask.DESCRIPTOR);
     registerExtension(taskId, BuildDirectiveElementsTask.DESCRIPTOR);
     registerExtension(taskId, BuildEnumMemberElementsTask.DESCRIPTOR);
     registerExtension(taskId, BuildExportNamespaceTask.DESCRIPTOR);
     registerExtension(taskId, BuildLibraryConstructorsTask.DESCRIPTOR);
     registerExtension(taskId, BuildLibraryElementTask.DESCRIPTOR);
     registerExtension(taskId, BuildPublicNamespaceTask.DESCRIPTOR);
-    registerExtension(taskId, BuildSourceClosuresTask.DESCRIPTOR);
+    registerExtension(taskId, BuildSourceExportClosureTask.DESCRIPTOR);
+    registerExtension(taskId, BuildSourceImportExportClosureTask.DESCRIPTOR);
     registerExtension(taskId, BuildTypeProviderTask.DESCRIPTOR);
     registerExtension(taskId, ComputeConstantDependenciesTask.DESCRIPTOR);
     registerExtension(taskId, ComputeConstantValueTask.DESCRIPTOR);
     registerExtension(taskId, ContainingLibrariesTask.DESCRIPTOR);
     registerExtension(taskId, DartErrorsTask.DESCRIPTOR);
     registerExtension(taskId, EvaluateUnitConstantsTask.DESCRIPTOR);
     registerExtension(taskId, GatherUsedImportedElementsTask.DESCRIPTOR);
     registerExtension(taskId, GatherUsedLocalElementsTask.DESCRIPTOR);
     registerExtension(taskId, GenerateHintsTask.DESCRIPTOR);
     registerExtension(taskId, LibraryErrorsReadyTask.DESCRIPTOR);
@@ skipping 18 matching lines @@
    * Validate the given extension by throwing an [ExtensionError] if it is not a
    * valid domain.
    */
   void _validateTaskExtension(Object extension) {
     if (extension is! TaskDescriptor) {
       String id = taskExtensionPoint.uniqueIdentifier;
       throw new ExtensionError('Extensions to $id must be a TaskDescriptor');
     }
   }
 }