Have loaders return resolved Promises in non-browser environments

addon/loaders/css.js

@@ -1,5 +1,5 @@
 import RSVP from 'rsvp';
-import { createLoadElement } from './utilities';
+import { createLoadElement, nodeLoader } from './utilities';
 
 /**
  * Default loader function for CSS assets. Loads them by inserting a link tag
@@ -13,7 +13,7 @@ import { createLoadElement } from './utilities';
  * @param {String} uri
  * @return {Promise}
  */
-export default function css(uri) {
+export default nodeLoader(function css(uri) {
   return new RSVP.Promise((resolve, reject) => {
     // Try using the default onload/onerror handlers...
     const link = createLoadElement('link', resolve, reject);
@@ -45,4 +45,4 @@ export default function css(uri) {
 
     setTimeout(checkSheetLoad);
   });
-}
+});

addon/loaders/js.js

@@ -1,5 +1,5 @@
 import RSVP from 'rsvp';
-import { createLoadElement } from './utilities';
+import { createLoadElement, nodeLoader } from './utilities';
 
 /**
  * Default loader function for JS assets. Loads them by inserting a script tag
@@ -9,12 +9,12 @@ import { createLoadElement } from './utilities';
  * @param {String} uri
  * @return {Promise}
  */
-export default function js(uri) {
+export default nodeLoader(function js(uri) {
   return new RSVP.Promise((resolve, reject) => {
     const script = createLoadElement('script', resolve, reject);
 
     script.src = uri;
 
     document.head.appendChild(script);
   });
-}
+});

addon/loaders/utilities.js

@@ -1,10 +1,14 @@
+import RSVP from 'rsvp';
+
+const isBrowser = typeof window !== 'undefined' && typeof document !== 'undefined';
+
 /**
  * Creates a DOM element with the specified onload and onerror handlers.
  *
  * @method createLoadElement
  * @param {String} tag
  * @param {Function} load
- * @paeam {Function} error
+ * @param {Function} error
  * @return {HTMLElement} el
  */
 export function createLoadElement(tag, load, error) {
@@ -15,3 +19,21 @@ export function createLoadElement(tag, load, error) {
 
   return el;
 }
+
+/**
+ * Creates a loader function that is compatible with Node environments (such as
+ * FastBoot). If we're in the browser, we'll use the passed in loader function,
+ * but when in Node, we'll just return a Promise that resolves (we assume assets
+ * will be pre-loaded).
+ *
+ * @method nodeLoader
+ * @param {Function} loader
+ * @return {Function}
+ */
+export function nodeLoader(loader) {
+  if (isBrowser) {
+    return loader;
+  } else {
+    return () => RSVP.resolve();
+  }
+}