FBJS: Fix getActiveElement w/o document

Summary:
Brings back #59.

src/core/dom/__tests__/getActiveElement-test.js

@@ -0,0 +1,22 @@
+/**
+ * Copyright 2013-2015, 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.
+ *
+ */
+
+'use strict';
+
+jest.dontMock('getActiveElement');
+
+var getActiveElement = require('getActiveElement');
+
+describe('getActiveElement', () => {
+  it('returns body when there is no activeElement', () => {
+    var element = getActiveElement();
+    expect(element.tagName).toEqual('BODY');
+  });
+});

src/core/dom/getActiveElement.js

@@ -10,13 +10,19 @@
  * @typechecks
  */
 
+/* eslint-disable fb-www/typeof-undefined */
+
 /**
  * Same as document.activeElement but wraps in a try-catch block. In IE it is
  * not safe to call document.activeElement if there is nothing focused.
  *
- * The activeElement will be null only if the document body is not yet defined.
+ * The activeElement will be null only if the document or document body is not
+ * yet defined.
  */
 function getActiveElement() /*?DOMElement*/ {
+  if (typeof document === 'undefined') {
+    return null;
+  }
   try {
     return document.activeElement || document.body;
   } catch (e) {