Tweak ADD_PART_OF quick fix to support both new/old task models.

pkg/analysis_server/lib/src/services/correction/fix_internal.dart

Index: pkg/analysis_server/lib/src/services/correction/fix_internal.dart
diff --git a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
index 9dc36d0c0caf99febf76ca74f186ab79bcefc6f8..cb695b8422aa497a16d42a805f9c1250492e78c1 100644
--- a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
+++ b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
@@ -413,8 +413,12 @@ class FixProcessor {
     if (node is SimpleStringLiteral && node.parent is PartDirective) {
       PartDirective directive = node.parent;
       Source partSource = directive.source;
-      CompilationUnit partUnit =
-          context.getResolvedCompilationUnit2(partSource, unitSource);
+      CompilationUnit partUnit;
+      if (AnalysisEngine.instance.useTaskModel) {
+        partUnit = context.getResolvedCompilationUnit2(partSource, partSource);
+      } else {
+        partUnit = context.getResolvedCompilationUnit2(partSource, unitSource);
+      }
       if (partUnit != null) {
         CorrectionUtils partUtils = new CorrectionUtils(partUnit);
         CorrectionUtils_InsertDesc desc = partUtils.getInsertDescTop();