Implement ElementHandle.attribute() method (#543)

This patch implements ElementHandle.attribute() method to fetch a value of
element's attribute.

docs/api.md

@@ -106,6 +106,7 @@
     + [frame.waitForFunction(pageFunction[, options, ...args])](#framewaitforfunctionpagefunction-options-args)
     + [frame.waitForSelector(selector[, options])](#framewaitforselectorselector-options)
   * [class: ElementHandle](#class-elementhandle)
+    + [elementHandle.attribute(key)](#elementhandleattributekey)
     + [elementHandle.click([options])](#elementhandleclickoptions)
     + [elementHandle.dispose()](#elementhandledispose)
     + [elementHandle.evaluate(pageFunction, ...args)](#elementhandleevaluatepagefunction-args)
@@ -1189,6 +1190,21 @@ puppeteer.launch().then(async browser => {
 
 ElementHandle prevents DOM element from garbage collection unless the handle is [disposed](#elementhandledispose). ElementHandles are auto-disposed when their origin frame gets navigated.
 
+#### elementHandle.attribute(key)
+- `key` <string> the name the attribute of this Element.
+- returns: <[Promise]>
+
+```js
+const puppeteer = require('puppeteer');
+
+puppeteer.launch().then(async browser => {
+  const page = await browser.newPage();
+  await page.goto('https://google.com');
+  const inputElement = await page.$('input');
+  const inputType = await inputElement.attribute('type');
+});
+```
+
 #### elementHandle.click([options])
 - `options` <[Object]>
   - `button` <[string]> `left`, `right`, or `middle`, defaults to `left`.

lib/ElementHandle.js

@@ -103,6 +103,15 @@ class ElementHandle {
     const objectId = this._remoteObject.objectId;
     return this._client.send('DOM.setFileInputFiles', { objectId, files });
   }
+
+  /**
+   * @param {!<string} key
+   * @return {!Promise}
+   */
+  async attribute(key) {
+    return await this.evaluate((element, key) => element.getAttribute(key), key);
+  }
+
 }
 
 module.exports = ElementHandle;

test/test.js

@@ -1180,6 +1180,12 @@ describe('Page', function() {
       }
       expect(error.message).toContain('ElementHandle is disposed');
     }));
+    it('should return attribute', SX(async function() {
+      await page.setContent('<section id="testAttribute">43543</section>');
+      const element = await page.$('section');
+      expect(element).toBeTruthy();
+      expect(await element.attribute('id')).toBe('testAttribute');
+    }));
   });
 
   describe('ElementHandle.click', function() {