IssueTracker

It appears that dev_appserver.py attempts to run apps/services even if their .yaml file contains the `env: flex` configuration. Of course, with unexpected results and typically obscure failures, since, as the standard env documentation mentions:

    Note: dev_appserver.py does not run in the App Engine flexible environment.

However a note along the same line is NOT present in the flexible environment docs, whcih is normally the one followed by the flexible environment users.

IMHO it'd be of great help to avoid confusion if dev_appserver.py would check if the configuration is a flexible environment one and immediately bail with a clear indication of the reason, before attempting to process other parts of a flexible app configuration which may be invalid/behave differently on the standard environment and thus cause obscure failures.

Sample related SO posts:

- https://stackoverflow.com/questions/49201561/google-app-engine-warning-the-cloud-sdk-no-longer-ships-runtimes-for-php-5-4
- https://stackoverflow.com/questions/45132686/google-cloud-dev-appserver-py-unable-to-host-laravel-project-locally/45865803#45865803
- https://stackoverflow.com/questions/37840809/dev-appserver-py-error-when-trying-to-deploy-to-google-appengine/37864869#37864869