packager: buck worker: output assets

Reviewed By: davidaurelio

Differential Revision: D4962816

fbshipit-source-id: 4ce4eacb28528c96829ac35134aff18a72063370

packages/metro-bundler/src/ModuleGraph/types.flow.js

@@ -142,9 +142,10 @@ export type LibraryOptions = {|
 |};
 
 export type Base64Content = string;
+export type AssetContentsByPath = {[destFilePath: string]: Base64Content};
 
 export type Library = {|
   files: Array<TransformedFile>,
   /* cannot be a Map because it's JSONified later on */
-  assets: {[destFilePath: string]: Base64Content},
+  assets: AssetContentsByPath,
 |};