diff --git a/packages/metro-bundler/src/ModuleGraph/worker.js b/packages/metro-bundler/src/ModuleGraph/worker.js
index 906f39fd..ec1635c4 100644
--- a/packages/metro-bundler/src/ModuleGraph/worker.js
+++ b/packages/metro-bundler/src/ModuleGraph/worker.js
@@ -21,4 +21,4 @@ import type {WorkerFnWithIO} from './worker/wrap-worker-fn';
 exports.optimizeModule =
   (wrapWorkerFn(optimizeModule): WorkerFnWithIO<OptimizationOptions>);
 exports.transformModule =
-  (wrapWorkerFn(transformModule): WorkerFnWithIO<TransformOptions>);
+  (wrapWorkerFn(transformModule): WorkerFnWithIO<TransformOptions<{+retainLines?: boolean}>>);
diff --git a/packages/metro-bundler/src/ModuleGraph/worker/transform-module.js b/packages/metro-bundler/src/ModuleGraph/worker/transform-module.js
index 81be5da8..809ba72c 100644
--- a/packages/metro-bundler/src/ModuleGraph/worker/transform-module.js
+++ b/packages/metro-bundler/src/ModuleGraph/worker/transform-module.js
@@ -33,11 +33,11 @@ import type {
 } from '../types.flow';
 import type {Ast} from 'babel-core';
 
-export type TransformOptions = {|
+export type TransformOptions<ExtraOptions> = {|
   filename: string,
   hasteImpl?: HasteImpl,
   polyfill?: boolean,
-  transformer: Transformer<*>,
+  transformer: Transformer<ExtraOptions>,
   variants?: TransformVariants,
 |};
 
@@ -56,7 +56,7 @@ const ASSET_EXTENSIONS = new Set(defaults.assetExts);
 
 function transformModule(
   content: Buffer,
-  options: TransformOptions,
+  options: TransformOptions<{+retainLines?: boolean}>,
 ): TransformedSourceFile {
   if (ASSET_EXTENSIONS.has(path.extname(options.filename).substr(1))) {
     return transformAsset(content, options);
@@ -143,9 +143,9 @@ function transformJSON(json, options): TransformedSourceFile {
   return {type: 'code', details: result};
 }
 
-function transformAsset(
+function transformAsset<ExtraOptions: {}>(
   content: Buffer,
-  options: TransformOptions,
+  options: TransformOptions<ExtraOptions>,
 ): TransformedSourceFile {
   return {
     details: {