build(browserstack): add a Gulp task to use Browser Stack locally

Closes #5116

DEVELOPER.md

@@ -189,6 +189,27 @@ tests respectively.
 **Note**: **watch mode** needs symlinks to work, so if you're using windows, ensure you have the
 rights to built them in your operating system.
 
+### Unit tests with Sauce Labs or Browser Stack
+
+First, in a terminal, create a tunnel with [Sauce Connect](https://docs.saucelabs.com/reference/sauce-connect/) or [Browser Stack Local](https://www.browserstack.com/local-testing#command-line), and valid credentials.  
+
+Then, in another terminal:
+ - Define the credentials as environment variables, e.g.:
+```
+export SAUCE_USERNAME='my_user'; export SAUCE_ACCESS_KEY='my_key';
+export BROWSER_STACK_USERNAME='my_user'; export BROWSER_STACK_ACCESS_KEY='my_key';
+```
+ - Then run `gulp test.unit.js.(saucelabs|browserstack) --browsers=option1,option2,..,optionN`  
+The options are any mix of browsers and aliases which are defined in the [browser-providers.conf.js](https://github.com/angular/angular/blob/master/browser-providers.conf.js) file.  
+They are case insensitive, and the `SL_` or `BS_` prefix must not be added for browsers.
+
+Some examples of commands:
+```
+gulp test.unit.js.saucelabs --browsers=Safari8,ie11  //run in Sauce Labs with Safari 8 and IE11
+gulp test.unit.js.browserstack --browsers=Safari,IE  //run in Browser Stack with Safari 7, Safari 8, Safari 9, IE 9, IE 10 and IE 11
+gulp test.unit.js.saucelabs --browsers=IOS,safari8,android5.1  //run in Sauce Labs with iOS 7, iOS 8, iOs 9, Safari 8 and Android 5.1
+```
+
 ### E2E tests
 
 1. `$(npm bin)/gulp build.js.cjs` (builds benchpress and tests into `dist/js/cjs` folder).