fix page.injectFile and add test. (puppeteer#52)

This line within `injectFile` wasn't doing much of anything: 

```js
let expression = fs.readFile(filePath, 'utf8', (err, data) => callback({err, data}));
```

* That's fixed.
* A path error in examples/features.js is fixed.
* Test added for injectFile.

examples/features.js

@@ -19,7 +19,7 @@ var Browser = require('../lib/Browser');
 var browser = new Browser();
 
 browser.newPage().then(async page => {
-    var modernizrPath = path.join('..', 'third_party', 'phantomjs', 'examples', 'modernizr.js');
+    var modernizrPath = path.join(__dirname, '../third_party/phantomjs/examples/modernizr.js');
     await page.injectFile(modernizrPath);
     page.on('consolemessage', console.log);
     await page.evaluate(detectFeatures);

lib/Page.js

@@ -128,11 +128,13 @@ class Page extends EventEmitter {
    * @return {!Promise}
    */
   async injectFile(filePath) {
-    let callback;
-    let promise = new Promise(fulfill => callback = fulfill);
-    let expression = fs.readFile(filePath, 'utf8', (err, data) => callback({err, data}));
-    await promise;
-    return this._client.send('Runtime.evaluate', { expression, returnByValue: true });
+    const contents = await new Promise((resolve, reject) => {
+      fs.readFile(filePath, 'utf8', (err, data) => {
+        if (err) return reject(err);
+        resolve(data);
+      });
+    });
+    return this._client.send('Runtime.evaluate', { expression: contents, returnByValue: true });
   }
 
   /**

test/assets/injectedfile.js

@@ -0,0 +1 @@
+__injected = 42;

test/test.js

@@ -104,6 +104,15 @@ describe('Puppeteer', function() {
     }));
   });
 
+  describe('Page.injectFile', function() {
+    it('should fail when navigating to bad url', SX(async function() {
+      const helloPath = path.join(__dirname, './assets/injectedfile.js');
+      await page.injectFile(helloPath);
+      const result = await page.evaluate(() => __injected);
+      expect(result).toBe(42);
+    }));
+  });
+
   describe('Frame.evaluate', function() {
     let FrameUtils = require('./frame-utils');
     it('should have different execution contexts', SX(async function() {