Refactor out strip source map fn

Summary: Remove the duplicate function and centralise it as nikoant suggested.

Reviewed By: fabiomassimo

Differential Revision: D29548480

fbshipit-source-id: 3e931cc88198415017c557c6b7c81cb35c3f22c9

desktop/pkg-lib/src/runBuild.ts

@@ -13,6 +13,7 @@ import path from 'path';
 import fs from 'fs-extra';
 import {getInstalledPluginDetails} from 'flipper-plugin-lib';
 import {FileStore} from 'metro-cache';
+import stripSourceMapComment from './stripSourceMap';
 import os from 'os';
 
 let metroDir: string | undefined;
@@ -37,18 +38,6 @@ type Options = {
   sourceMapPath?: string | undefined;
 };
 
-// Metro erroneously adds source map comments to the bottom of the file
-// which break checksums on CI environments where paths change and are generally
-// undesired. We manually strip the comment here and write the file back.
-async function stripSourceMapComment(out: string) {
-  const lines = (await fs.readFile(out, 'utf-8')).split(os.EOL);
-  const lastLine = lines[lines.length - 1];
-  if (lastLine.startsWith('//# sourceMappingURL=')) {
-    console.log(`Updating ${out} to remove sourceMapURL= comment.`);
-    await fs.writeFile(out, lines.slice(0, lines.length - 1).join(os.EOL));
-  }
-}
-
 export default async function bundlePlugin(
   pluginDir: string,
   dev: boolean,