Support single client mode for DDS #1091
Description
The Dart Development Service expects that it is the only client of a VM service implementer in order to simplify state management. DDS enters this mode by invoking _yieldControlToDDS, at which point the VM service implementation should no longer accept connections while DDS is connected. If there is already a client connected to the VM service implementation, it should return an error so DDS knows to exit.
Currently there's simple extension in package:dwds which registers _yieldControlToDDS as a service extension and always returns Success. package:dwds should add support for single client mode to avoid situations where DDS and other VM service clients are both connected to DWDS.
cc/ @grouma