Fix navigation to about:blank

This patch fixes navigation to about:blank url.

docs/api.md

@@ -461,6 +461,9 @@ The `page.navigate` will throw an error if:
 - there's an SSL error (e.g. in case of self-signed certificates).
 - target URL is invalid.
 - the `timeout` is exceeded during navigation.
+- the main resource failed to load.
+
+> **NOTE** `page.navigate` either throw or return a main resource response. The only exception is navigation to `about:blank`, which would succeed and return `null`.
 
 #### page.pdf(options)
 - `options` <[Object]> Options object which might have the following properties:

lib/Page.js

@@ -242,7 +242,7 @@ class Page extends EventEmitter {
   /**
    * @param {string} html
    * @param {!Object=} options
-   * @return {!Promise<!Response>}
+   * @return {!Promise<?Response>}
    */
   async navigate(url, options) {
     const watcher = new NavigatorWatcher(this._client, options);
@@ -260,6 +260,8 @@ class Page extends EventEmitter {
     }
     await result;
     helper.removeEventListeners([listener]);
+    if (url === 'about:blank')
+      return null;
     let response = responses.get(this.mainFrame().url());
     if (!response)
       throw new Error('Failed to navigate: ' + url);

test/test.js

@@ -443,6 +443,10 @@ describe('Puppeteer', function() {
   });
 
   describe('Page.navigate', function() {
+    it('should navigate to about:blank', SX(async function() {
+      let response = await page.navigate('about:blank');
+      expect(response).toBe(null);
+    }));
     it('should fail when navigating to bad url', SX(async function() {
       let error = null;
       try {