Migrator: do not generate edit details twice on rerun

Fixes #44086

Change-Id: I99a23e5922a716bbd0c83e7c0f620686840ccd4c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/172300
Reviewed-by: Paul Berry <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>

Author: srawlins

pkg/nnbd_migration/lib/src/front_end/non_nullable_fix.dart

@@ -122,7 +122,7 @@ class NonNullableFix {
   /// file, and the analysis_options.yaml file, each only if necessary.
   ///
   /// [neededPackages] is a map whose keys are the names of packages that should
-  /// be dependend upon by the package's pubspec, and whose values are the
+  /// be depended upon by the package's pubspec, and whose values are the
   /// minimum required versions of those packages.
   void processPackage(Folder pkgFolder, Map<String, Version> neededPackages) {
     var pubspecFile = pkgFolder.getChildAssumingFile('pubspec.yaml');
@@ -156,7 +156,6 @@ class NonNullableFix {
   Future<MigrationState> rerun() async {
     reset();
     var state = await rerunFunction();
-    await state.refresh(_logger);
     return state;
   }
 

pkg/nnbd_migration/lib/src/preview/preview_site.dart

@@ -364,7 +364,9 @@ class PreviewSite extends Site
 
   Future<void> rerunMigration() async {
     migrationState = await rerunFunction();
-    reset();
+    if (!migrationState.hasErrors) {
+      reset();
+    }
   }
 
   void reset() {