Add sourceFactory getter to AnalysisSession

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

Index: pkg/analysis_server/lib/src/computer/import_elements_computer.dart
diff --git a/pkg/analysis_server/lib/src/computer/import_elements_computer.dart b/pkg/analysis_server/lib/src/computer/import_elements_computer.dart
index f9328f4351017bd7e3b8ac8be9cae3af4abaf707..c30639db624cc769d0d17e934eb579383e3279b2 100644
--- a/pkg/analysis_server/lib/src/computer/import_elements_computer.dart
+++ b/pkg/analysis_server/lib/src/computer/import_elements_computer.dart
@@ -53,7 +53,7 @@ class ImportElementsComputer {
     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) {