Add sourceFactory getter to AnalysisSession

pkg/analysis_server/lib/src/computer/import_elements_computer.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 
 import 'package:analysis_server/protocol/protocol_generated.dart';
 import 'package:analyzer/dart/analysis/results.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/ast_factory.dart';
@@ skipping 35 matching lines @@
 
   /**
    * Create the edits that will cause the list of [importedElements] to be
    * imported into the library at the given [path].
    */
   Future<SourceChange> createEdits(
       List<ImportedElements> importedElementsList) async {
     List<ImportedElements> filteredImportedElements =
         _filterImportedElements(importedElementsList);
     LibraryElement libraryElement = libraryResult.libraryElement;
-    SourceFactory sourceFactory = libraryElement.context.sourceFactory;
+    SourceFactory sourceFactory = libraryResult.session.sourceFactory;
     List<ImportDirective> existingImports = <ImportDirective>[];
     for (var directive in libraryResult.unit.directives) {
       if (directive is ImportDirective) {
         existingImports.add(directive);
       }
     }
 
     DartChangeBuilder builder = new DartChangeBuilder(libraryResult.session);
     await builder.addFileEdit(libraryResult.path,
         (DartFileEditBuilder builder) {
@@ skipping 334 matching lines @@
     namesToShow.add(name);
   }
 
   /**
    * Record that the given [name] needs to be removed from hide combinators.
    */
   void unhide(String name) {
     namesToUnhide.add(name);
   }
 }