Do not export null as module.exports in amd.js. Resolves #2332. (#2334)

* Do not export null as module.exports in amd.js. Resolves #2332.

* Make named-exports and system-core resilient to _export(null)

* Comment

* Guy's feedback

src/extras/named-exports.js

@@ -30,7 +30,7 @@
       // hook the _export function to note the default export
       var defaultExport, hasDefaultExport = false;
       var declaration = registerDeclare.call(this, function (name, value) {
-        if (typeof name === 'object' && name.__useDefault)
+        if (typeof name === 'object' && name && name.__useDefault)
           defaultExport = name.default, hasDefaultExport = true;
         else if (name === 'default')
           defaultExport = value;

src/system-core.js

@@ -112,7 +112,7 @@ export function getOrCreateLoad (loader, id, firstParentUrl) {
           }
         }
 
-        if (name.__esModule) {
+        if (name && name.__esModule) {
           ns.__esModule = name.__esModule;
         }
       }

test/browser/amd.js

@@ -83,6 +83,13 @@ suite('AMD tests', function () {
     });
   });
 
+  // https://github.com/systemjs/systemjs/issues/2332
+  test('loading an AMD module that sets module.exports to null', function () {
+    return System.import('fixtures/amd-null-module.js').then(function (m) {
+      assert.equal(m.default, null);
+    });
+  });
+
   test('Throws an error when define() is called incorrectly', () => {
     try {
       define("strings are invalid amd modules");

test/fixtures/browser/amd-null-module.js

@@ -0,0 +1,3 @@
+define([], function () {
+  return null;
+});