Fix invalid syntax with deferRendering.

An earlier refactor (to avoid floating the passed in `promise`)
introduced invalid syntax (`Promise.all(firstThing, secondThing)`), this
fixes the issue and adds tests ensuring we don't regress.

Author: Robert Jackson

src/fastboot-info.js

@@ -28,7 +28,7 @@ module.exports = class FastBootInfo {
   }
 
   deferRendering(promise) {
-    this.deferredPromise = Promise.all(this.deferredPromise, promise);
+    this.deferredPromise = Promise.all([this.deferredPromise, promise]);
   }
 
   /*

test/fastboot-info-test.js

@@ -3,6 +3,16 @@ var FastBootInfo = require('./../src/fastboot-info.js');
 var FastBootResponse = require('./../src/fastboot-response.js');
 var FastBootRequest = require('./../src/fastboot-request.js');
 
+function delayFor(ms) {
+  let promise = new Promise(resolve => {
+    setTimeout(() => {
+      resolve();
+    }, ms);
+  });
+
+  return promise;
+}
+
 describe('FastBootInfo', function() {
   var response;
   var request;
@@ -37,4 +47,25 @@ describe('FastBootInfo', function() {
   it('has metadata', function() {
     expect(fastbootInfo.metadata).to.deep.equal(metadata);
   });
+
+  it('can use deferRendering', async function() {
+    let steps = [];
+
+    steps.push('deferRendering called');
+
+    fastbootInfo.deferRendering(delayFor(10).then(() => steps.push('first delay completed')));
+    fastbootInfo.deferRendering(delayFor(10).then(() => steps.push('second delay completed')));
+    fastbootInfo.deferRendering(delayFor(20).then(() => steps.push('third delay completed')));
+    fastbootInfo.deferRendering(delayFor(15).then(() => steps.push('fourth delay completed')));
+
+    await fastbootInfo.deferredPromise;
+
+    expect(steps).to.deep.equal([
+      'deferRendering called',
+      'first delay completed',
+      'second delay completed',
+      'fourth delay completed',
+      'third delay completed',
+    ]);
+  });
 });