Immutable props fix (fixes #2433)

[claviska]

This fixes a longstanding bug where the warning for immutable prop changes doesn't get shown. It also adds tests to ensure the warning is emitted at the correct times.

Side note: this will probably confuse people once it gets merged because a lot of projects are likely changing props without the { mutable: true } flag. I've already noticed a few in Stencil Router. 😆

[NRaf]

@claviska Just updated to Stencil 2.4.0 and noticed this warning starting to come through my unit tests. I'm not actually mutating any props through my component logic, however, I do mutate props in my unit tests. Is there any way to disable the warning during tests? Or is there a recommended alternative?

[allinne]

My unit tests pass even in 2.0.1 with an error:
@Prop() "state" on <my-component> is immutable but was modified from within the component.
The test looks like:

const myComponent = new MyComponent();
myComponent.state = 'error';

To pass the tests I needed make all props mutable but it is not the right way. Please, clarify this moment.