document the spyOnDependency method

doc/development/testing_guide/frontend_testing.md

@@ -126,13 +126,51 @@ it('tests a promise rejection', (done) => {
 });
 ```
 
-#### Stubbing
+#### Stubbing and Mocking
 
-For unit tests, you should stub methods that are unrelated to the current unit you are testing.
-If you need to use a prototype method, instantiate an instance of the class and call it there instead of mocking the instance completely.
+Jasmine provides useful helpers `spyOn`, `spyOnProperty`, `jasmine.createSpy`,
+and `jasmine.createSpyObject` to facilitate replacing methods with dummy
+placeholders, and recalling when they are called and the arguments that are
+passed to them. These tools should be used liberally, to test for expected
+behavior, to mock responses, and to block unwanted side effects (such as a
+method that would generate a network request or alter `window.location`). The
+documentation for these methods can be found in the [jasmine introduction page](https://jasmine.github.io/2.0/introduction.html#section-Spies).
 
-For integration tests, you should stub methods that will effect the stability of the test if they
-execute their original behaviour. i.e. Network requests.
+Sometimes you may need to spy on a method that is directly imported by another
+module. GitLab has a custom `spyOnDependency` method which utilizes
+[babel-plugin-rewire](https://github.com/speedskater/babel-plugin-rewire) to
+achieve this.  It can be used like so:
+
+```javascript
+// my_module.js
+import { visitUrl } from '~/lib/utils/url_utility';
+
+export default function doSomething() {
+  visitUrl('/foo/bar');
+}
+
+// my_module_spec.js
+import doSomething from '~/my_module';
+
+describe('my_module', () => {
+  it('does something', () => {
+    const visitUrl = spyOnDependency(doSomething, 'visitUrl');
+
+    doSomething();
+    expect(visitUrl).toHaveBeenCalledWith('/foo/bar');
+  });
+});
+```
+
+Unlike `spyOn`, `spyOnDependency` expects its first parameter to be the default
+export of a module who's import you want to stub, rather than an object which
+contains a method you wish to stub (if the module does not have a default
+export, one is be generated by the babel plugin). The second parameter is the
+name of the import you wish to change. The result of the function is a Spy
+object which can be treated like any other jasmine spy object.
+
+Further documentation on the babel rewire pluign API can be found on
+[its repository Readme doc](https://github.com/speedskater/babel-plugin-rewire#babel-plugin-rewire).
 
 ### Vue.js unit tests