registerExtension#
Register a ServiceExtensionHandler
that will be invoked in this isolate
for method. NOTE: Service protocol extensions must be registered
in each isolate.
NOTE: method must begin with 'ext.' and you should use the following
structure to avoid conflicts with other packages: 'ext.package.command'.
That is, immediately following the 'ext.' prefix, should be the registering
package name followed by another period ('.') and then the command name.
For example: 'ext.dart.io.getOpenFiles'.
Because service extensions are isolate specific, clients using extensions must always include an 'isolateId' parameter with each RPC.