Write not-found.json to ExportedApi (#8252)

Author: jonasfj

app/lib/package/api_export/api_exporter.dart

@@ -147,6 +147,15 @@ class ApiExporter {
 
     await synchronizePackageNameCompletionData();
 
+    await _api.notFound.write({
+      'error': {
+        'code': 'NotFound',
+        'message': 'Package or version requested could not be found.',
+      },
+      'code': 'NotFound',
+      'message': 'Package or version requested could not be found.',
+    });
+
     await _api.garbageCollect(allPackageNames);
   }
 

app/lib/package/api_export/exported_api.dart

@@ -66,6 +66,15 @@ final class ExportedApi {
         Duration(hours: 8),
       );
 
+  /// Interface for writing `/api/not-found.json` which is what the bucket will
+  /// use as 404 response when serving a website.
+  ExportedJsonFile<Map<String, Object?>> get notFound =>
+      ExportedJsonFile<Map<String, Object?>>._(
+        this,
+        '/not-found.json',
+        Duration(minutes: 10),
+      );
+
   /// Run garbage collection on the bucket.
   ///
   /// This will remove all packages from `latest/` and `<runtimeVersion>/`,