Unfortunately, debugging a plugin is not well supported at this point. The server is typically run as a sub-process by a client. Some clients provide a way to add command-line arguments when invoking the server, others don't. To make matters worse, each plugin is run in a separate isolate.
Nevertheless, there are a few ways to get some information about what's going on in a plugin. Those are outlined below. If you know of other useful techniques, or if you have a request for better support, please let us know.
The analysis server has the ability to host a set of web pages that describe the current state of the server. One of the pieces of information available through those pages is a list of the plugins that are currently being run.
The server does not host these pages unless it has been requested to do so. You can request it by passing a command-line argument to the server when it is being started. The argument should be similar to --port=10000 (any valid port number will work). You can then point your browser to http://localhost:10000/status.
If you‘re using IntelliJ as your client, there is a gear icon on the Dart Analysis view’s header that can be used to open the status pages.
The plugin information can be displayed by clicking on “Plugins” in the list on the left.
The analysis server has the ability to log debugging data to a file. For historic reasons this file is referred to as the instrumentation log. The data primarily consists of a record of the communications between the server and both the client that started it and any plugins that the server is running.
The server does not write to this file unless it has been requested to do so. You can request it by passing a command-line argument to the server when it is being started. The argument should be similar to --instrumentation-log-file=/path/to/file.txt.
You cannot use the print function to get debugging information because server is run by the client in a child process, and hence doesn't have the ability to write to the console.
The closest approximation is for the plugin to send notifications to the server that will be written to the instrumentation log file. Currently, the best choice for this is the plugin.error notification. Just be sure that isFatal has a value of false.
If the client you're using allows you to pass command-line flags to the VM, then you can also run the analysis server under the Observatory. Pass in both --observe and --pause-isolates-on-start, then point your browser to http://localhost:8181. To learn more, see the observatory documentation.
If you're using IntelliJ as your client, open the “Registry...” dialog and edit the entry named “dart.server.vm.options”.