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/Graph.js

@@ -13,6 +13,7 @@
 const emptyFunction = require('fbjs/lib/emptyFunction');
 const invariant = require('fbjs/lib/invariant');
 const memoize = require('async/memoize');
+const emptyModule = require('./module').empty;
 const nullthrows = require('fbjs/lib/nullthrows');
 const queue = require('async/queue');
 const seq = require('async/seq');
@@ -49,9 +50,6 @@ type Async$Queue<T, C> = {
 type LoadQueue =
   Async$Queue<{id: string, parent: ?string}, Callback<File, Array<string>>>;
 
-const createParentModule =
-  () => ({file: {code: '', type: 'script', path: ''}, dependencies: []});
-
 const NO_OPTIONS = {};
 
 exports.create = function create(resolve: ResolveFn, load: LoadFn): GraphFn {
@@ -101,7 +99,7 @@ exports.create = function create(resolve: ResolveFn, load: LoadFn): GraphFn {
 };
 
 function createGraphHelpers(loadQueue, skip) {
-  const modules = new Map([[null, createParentModule()]]);
+  const modules = new Map([[null, emptyModule()]]);
 
   function collect(
     path = null,