Allow passing in of Partial feature flags (#9652)

Author: benjervis

packages/core/types-internal/scripts/build-ts.js

@@ -11,5 +11,6 @@ contents = contents.replace(
 );
 contents = contents.replace(/\$ReadOnlyMap/g, 'ReadonlyMap');
 contents = contents.replace(/\$ReadOnlySet/g, 'ReadonlySet');
+contents = contents.replace(/\$Partial/g, 'Partial');
 
 fs.writeFileSync(typesPath, contents);

packages/core/types-internal/src/index.js

@@ -385,7 +385,7 @@ export type InitialParcelOptionsInternal<WorkerFarm> = {|
     resolveFrom: FilePath,
   |}>,
 
-  +featureFlags?: FeatureFlags,
+  +featureFlags?: $Partial<FeatureFlags>,
 
   // throwErrors
   // global?