Add flow types for output modules

Summary: Adds flow types for output functionality for easier maintenance and interop with new code

Reviewed By: matryoshcow

Differential Revision: D4211863

fbshipit-source-id: 591407d3a6d49536054ae94ba31125c18a1e1fa1

local-cli/bundle/output/unbundle/write-sourcemap.js

@@ -5,12 +5,18 @@
  * This source code is licensed under the BSD-style license found in the
  * LICENSE file in the root directory of this source tree. An additional grant
  * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
  */
 'use strict';
 
 const writeFile = require('../writeFile');
 
-function writeSourcemap(fileName, contents, log) {
+function writeSourcemap(
+  fileName: string,
+  contents: string,
+  log: (x: string) => void,
+): Promise<> {
   if (!fileName) {
     return Promise.resolve();
   }