Append `sourceMappingURL` in bundle building function

Summary:
We appended a `sourceMappingURL` in the same place where we write out the files. This will break output that is not a plain text file, like Random Access Bundles.
This moves the corresponding logic into the function that builds the bundle.

Reviewed By: cpojer

Differential Revision: D5061039

fbshipit-source-id: 17fadb5a687c8d4b1f29439e8bf946bae58eb2d9

packages/metro-bundler/src/ModuleGraph/output/as-plain-bundle.js

@@ -16,7 +16,7 @@ const {addModuleIdsToModuleWrapper} = require('./util');
 import type {OutputFn} from '../types.flow';
 
 module.exports = (
-  (modules, filename, idForPath) => {
+  (modules, filename, idForPath, sourceMapPath) => {
     let code = '';
     let line = 0;
     const sections = [];
@@ -37,6 +37,10 @@ module.exports = (
       line += countLines(moduleCode);
     }
 
+    if (sourceMapPath) {
+      code += `/*# sourceMappingURL=${sourceMapPath}*/`;
+    }
+
     return {code, map: createIndexMap({file: filename, sections})};
   }: OutputFn);
 

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

@@ -72,8 +72,9 @@ export type Module = {|
 
 export type OutputFn = (
   modules: Iterable<Module>,
-  filename?: string,
+  filename: string,
   idForPath: IdForPathFn,
+  sourceMapPath?: string,
 ) => OutputResult;
 
 type OutputResult = {|