Introduce DEBUG module

This patch re-introduces the DEBUG module to expose some of
the puppeteer's internals.

Currently, only the protocol message communication is exposed under
the 'puppeteer:protocol' namespace.

CONTRIBUTING.md

@@ -40,7 +40,7 @@ are used to test `phantom_shim`.
 
 To run puppeteer tests, use:
 ```
-npm run test-puppeteer
+npm run unit
 ```
 
 To run phantom-shim against phantomjs tests, use:
@@ -53,3 +53,15 @@ To run both puppeteer and phantom_shim tests, use:
 npm test
 ```
 
+## DEBUG module
+Puppeteer uses [debug](https://github.com/visionmedia/debug) module to expose some of it's inner guts under the `puppeteer` namespace.
+Try putting the following script in the `script.js` and running it via `DEBUG=* node script.js`:
+
+```js
+const {Browser} = require('puppeteer');
+const browser = new Browser();
+browser.newPage().then(async page => {
+  await page.navigate('https://example.com');
+  browser.close();
+});
+```

lib/Connection.js

@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+let debug = require('debug')('puppeteer:protocol');
 
 let EventEmitter = require('events');
 let WebSocket = require('ws');
@@ -46,6 +47,7 @@ class Connection extends EventEmitter {
   send(method, params = {}) {
     let id = ++this._lastId;
     let message = JSON.stringify({id, method, params});
+    debug('◀ SEND ' + message);
     this._ws.send(message);
     return new Promise((resolve, reject) => {
       this._callbacks.set(id, {resolve, reject, method});
@@ -56,6 +58,7 @@ class Connection extends EventEmitter {
    * @param {string} message
    */
   _onMessage(message) {
+    debug('RECV ► ' + message);
     let object = JSON.parse(message);
     if (object.id && this._callbacks.has(object.id)) {
       let callback = this._callbacks.get(object.id);

package.json

@@ -20,6 +20,7 @@
   "author": "The Chromium Authors",
   "license": "SEE LICENSE IN LICENSE",
   "dependencies": {
+    "debug": "^2.6.8",
     "extract-zip": "^1.6.5",
     "mime": "^1.3.4",
     "progress": "^2.0.0",