With this commit, props validation error messages will include a more
developer-friendly error message, avoiding the need to investigate in
the Developer Tools why a complex props structure is invalid.
We re-add the possibility to validate props when dev mode is active.
No change in the API right Now. The dev mode is activated via the
configure method of App class.
The point is to have visibility on the development of the owl2 features.
This commit reintroduces some tests keeping them skipped in order to fulfill that purpose.
There still are some missing tests though.