In
https://github.com/dart-lang/sdk/commit/df8bf384eb815cf38450cb50a0f4b62230fba217
a new functionality of the Dart VM Service Protocol has been introduced.

Clients connected to the Service Protocol are now able to expose
services that other clients (e.g. Observatory) can invoke through the
Service Protocol itself.

With these changes Flutter Tools register them self as a `reloadSources`
(a.k.a. HotReload) capable client.
Observatory is already listening for the clients which expose this
functionality and uses by default the service based version of
`reloadSources` when available, so requesting a HotReload from
Observatory will trigger the full Flutter HotReload.

Related https://github.com/dart-lang/sdk/issues/30023
Related https://github.com/flutter/flutter/pull/11229
Related https://github.com/flutter/flutter/pull/11256