Analyzer-plugin should allow extending refactoring

[rrousselGit]

A follow up to this conversation: https://groups.google.com/a/dartlang.org/forum/#!topic/analyzer-discuss/7mswT7KH1AU

The most important part of this discussion is:

As I'm sure you've figured out, when a client (like IntelliJ or VS Code) wants to make some functionality available to the user, it does so by using the client-server protocol defined by the Analysis Server API Specification to send a request to the server. If the server supports plugin participation in that request, it will then use the server-plugin protocol defined by the Analysis Server Plugin API Specification to send a request to the plugin.

Unfortunately, the plugin protocol does not currently support any APIs that would allow plugins to participate in refactoring requests.

I didn't realize that the plugin code still contained handler methods (handleEditGetAvailableRefactorings and handleEditGetRefactoring) related to refactorings. Those were added as part of an earlier exploration of what kind of refactoring support we might be able to add eventually, but they are not used. The requests they purport to handle don't exist, so server cannot send the requests, so those methods will never be invoked. They should have been removed. I'm sorry for the confusion their presence has caused.

I do hope that we can eventually return to the task of adding refactoring support for plugins, but until then I'm sorry to say that it just isn't possible.

A typical use-case is to extend the renaming refactor, so that multiple classes are edited simultaneously similarly to how a State from Flutter is renamed when StatefulWidget is refactored.