fix #9224 Warn when the type of a prop isn't a constructor

[cedric05]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[posva]

Thanks but this is missing many things: it should be a development only warning and it should contain tests 🙂

[cedric05]

@posva thanks for pointing out.
what would be correct place to write tests for above usecase. if not i will be writing in unit folder.
am i missing any thing else other than writing tests?

[posva]

look for other tests about warnings. Specifically the ones for props should help

[posva]

Closing in favor of #9241