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

packages/metro-bundler/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,

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

@@ -14,8 +14,7 @@ const defaults = require('../../defaults');
 const nullthrows = require('fbjs/lib/nullthrows');
 const parallel = require('async/parallel');
 const seq = require('async/seq');
-
-const {virtualModule} = require('./output/util');
+const virtualModule = require('./module').virtual;
 
 import type {
   Callback,

packages/metro-bundler/src/ModuleGraph/__tests__/ModuleGraph-test.js

@@ -32,6 +32,7 @@ describe('build setup', () => {
         file: {
           code: 'var __DEV__=true,__BUNDLE_START_TIME__=' +
             'this.nativePerformanceNow?nativePerformanceNow():Date.now();',
+          map: null,
           path: '',
           type: 'script',
         },

packages/metro-bundler/src/ModuleGraph/module.js

@@ -0,0 +1,28 @@
+/**
+ * Copyright (c) 2016-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * 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';
+
+
+import type {Module} from './types.flow';
+
+exports.empty = (): Module => virtual('');
+
+// creates a virtual module (i.e. not corresponding to a file on disk)
+// with the given source code.
+const virtual = exports.virtual = (code: string): Module => ({
+  dependencies: [],
+  file: {
+    code,
+    map: null,
+    path: '',
+    type: 'script',
+  },
+});

packages/metro-bundler/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',
-    },
-  };
-}

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

@@ -26,7 +26,7 @@ type Dependency = {|
 
 export type File = {|
   code: string,
-  map?: ?MappingsMap,
+  map: ?MappingsMap,
   path: string,
   type: CodeFileTypes,
 |};