For Google App Engine running Python, the command would be
dev_appserver.py app.yaml --host $IP --port $PORT --admin_host $IP --admin_port 8081
You can also specify the admin host/port. Since Cloud 9 allows access to 8081 and 8082, you can use either of those as your admin ports. For me, the admin console did not open with the Cloud9 preview, but did open in a new tab within my browser.
$IP and $PORT are both environment variables for Cloud 9, with the values of 0.0.0.0 and 8080 respectively.
Edit:
With the most recent gcloud update (March 2018), you don't have to change the IP or PORT, but you do need to figure out how to work around the host whitelisting issue.
My non-ideal workaround is to add a flag to not check for hosts --enable_host_checking=false
.
dev_appserver.py app.yaml --enable_host_checking=false
There's an unanswered Cloud 9 post around this issue (link to c9 forum). My guess is that you can add $C9_HOSTNAME as the host, but that doesn't quite work for me.
Interactive Console
If you want to use the interactive console you need to assign the admin port and also use the `--enable_console' flag.
dev_appserver.py app.yaml --enable_host_checking=false --admin_port 8081 --enable_console=true