Simplify File type: map is nullable, but not optional

Summary:
Simplifies the `File` type by making `map` a non-optional, but nullable property.
Also adds helper functions for empty/virtual modules

Reviewed By: jeanlauliac

Differential Revision: D5111580

fbshipit-source-id: 9cab6634a9bdb0522dc36aec2abccaef9cf35339

packager/src/ModuleGraph/output/util.js

@@ -10,6 +10,8 @@
  */
 'use strict';
 
+const virtualModule = require('../module').virtual;
+
 import type {IdForPathFn, Module} from '../types.flow';
 
 // Transformed modules have the form
@@ -77,17 +79,3 @@ exports.requireCallsTo = function* (
     yield virtualModule(`require(${idForPath(module.file)});`);
   }
 };
-
-// creates a virtual module (i.e. not corresponding to a file on disk)
-// with the given source code.
-exports.virtualModule = virtualModule;
-function virtualModule(code: string) {
-  return {
-    dependencies: [],
-    file: {
-      code,
-      path: '',
-      type: 'script',
-    },
-  };
-}