docs(test): Improve marble testing docs (#3991)

* docs(test): Added  import statement for TestScheduler
* docs(test): Make the first marble testing example more complete.

doc/marble-testing.md

@@ -7,25 +7,31 @@ We can test our _asynchronous_ RxJS code _synchronously_ and deterministically b
 > At this time the TestScheduler can only be used to test code that uses timers, like delay/debounceTime/etc (i.e. it uses AsyncScheduler with delays > 1). If the code consumes a Promise or does scheduling with AsapScheduler/AnimationFrameScheduler/etc it cannot be reliably tested with TestScheduler, but instead should be tested more traditionally. See the [Known Issues](#known-issues) section for more details.
 
 ```ts
-const testScheduler = new TestScheduler((actual, expected) => {
- // some how assert the two objects are equal
- // e.g. with chai `expect(actual).deep.equal(expected)`
+import { TestScheduler } from 'rxjs/testing';
+
+const scheduler = new TestScheduler((actual, expected) => {
+ // asserting the two objects are equal
+ // e.g. using chai.
+ expect(actual).deep.equal(expected);
 });
 
 // This test will actually run *synchronously*
-testScheduler.run(({ cold }) => {
- const input = cold('-a-b-c--------|');
- const output = input.pipe(
- debounceTime(5)
- );
- const expected = ' ----------c---|';
- expectObservable(output).toBe(expected);
+it('generate the stream correctly', () => {
+ scheduler.run(helpers => {
+ const { cold, expectObservable, expectSubscriptions } = helpers;
+ const e1 = cold('-a--b--c---|');
+ const subs = '^----------!';
+ const expected = '-a-----c---|';
+
+ expectObservable(e1.pipe(throttleTime(3, scheduler))).toBe(expected);
+ expectSubscriptions(e1.subscriptions).toBe(subs);
+ });
 });
 ```
 
 ## API
 
-The callback function you provide to `testScheduler.run(callback)` is called with an object that contains the helper functions you'll use to write your tests.
+The callback function you provide to `testScheduler.run(callback)` is called with `helpers` object that contains functions you'll use to write your tests.
 
 > When the code inside this callback is being executed, any operator that uses timers/AsyncScheduler (like delay, debounceTime, etc) will **automatically** use the TestScheduler instead, so that we have "virtual time". You do not need to pass the TestScheduler to them, like in the past.