Technically this is a behavior change, but any usage of "host:" or ":" should be considered invalid, since dialing a target with a trailing ":" attempts to dial port zero, which is never valid (and we always used to pass the target directly to `net.Dial`).
In https://github.com/grpc/grpc/pull/15618, the target format will be defined as: `host[:port]` -- not: `host[:[port]]`. (This is potentially subject to change.)
