docs: testing - highlight dispatchEvent()

[wardbell]

I spent hours trying to make Angular recognize the value entered into an input box of a form.

The solution - inputElement.dispatchEvent(newEvent('input')); - was hiding in Testing sample code but the guide did not speak to it. Obviously I knew the answer once long ago but had forgotten it.

I got no help searching the web.

I only remembered the answer when I looked at integration tests for the Angular Reactive Forms. Our readers should not have to work so hard to learn this technique.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

It devotes a new section to usage of dispatchEvent.

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] angular.io application / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

This critical technique was not covered (although examples of it were visible in several examples)

What is the new behavior?

Usage called out just after detectChanges()). Go to ~/guide/testing#dispatch-event`.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

[mary-poppins]

You can preview c47990d at https://pr22726-c47990d.ngbuilds.io/.

[mary-poppins]

You can preview 0b53b71 at https://pr22726-0b53b71.ngbuilds.io/.

[petebacondarwin]

I'm not quite sure that this is the correct conversational style for the guide?

[petebacondarwin]

It sort of implies that something is wrong with Angular.

[Foxandxss]

I agree that maybe the sentence is not needed. More like because it says "All I said is not what you want, so forget about it and then let me explain it good now"

But with all due respect, I do think that something is wrong with Angular. All of this shouldn't be needed and detectChanges by itself should be enough.

[wardbell]

I didn't intend to suggest there was something wrong with Angular but rather that we might not expect this. I will rephrase.

[wardbell]

@petebacondarwin I rephrased the objectionable passage.

[mary-poppins]

You can preview 971e298 at https://pr22726-971e298.ngbuilds.io/.

[petebacondarwin]

Blessings!

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