feat(cdk/testing): Consider adding a `TestElementPromise` class

[mmalerba]

Feature Description

@yjaaidi proposed this idea in #20871. We could create a subclass of Promise that would allow calling TestElement methods directly on the Promise. Example: https://stackblitz.com/edit/test-element-promise

Use Case

This would allow people to write cleaner test code, e.g.

await harness.host().getText();

Instead of:

await (await harness.host()).getText();
// or
await harness.host().then(host => host.getText());

The idea would need a design doc and discussion with the team before proceeding.

[crisbeto]

One thing we'll have to consider is that the Promise might be polyfilled which may cause issues with inheritance.

[edbzn]

@crisbeto As I read in the documentation the recommended way to use Harness is using the ES2017 async/await syntax, so is that a real issue?

[Splaktar]

While polyfills are certainly used in some test configurations, especially when i18n is involved, it's unlikely that there would be any need to polyfill Promise now that Angular only supports IE11 and other browsers that support native Promise.

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