"TypeError: Cannot read property 'then' of undefined" using 8.0.0-next.12 with latest VS Code stable

@dbaeumer I’m not sure whether this is expected to work yet (using 8.0.0-next.12 from npm with the newly published VS Code stable), but I’m seeing this issue when my server calls showMessage:

[12:39:38 PM] [CI] [Error] TypeError: Cannot read property 'then' of undefined
    at LanguageClient.showNotificationMessage ([/Users/danny/Dev/Dart-Code/node_modules/vscode-languageclient/lib/common/client.js:2256:1]())
    at LanguageClient.error ([/Users/danny/Dev/Dart-Code/node_modules/vscode-languageclient/lib/common/client.js:2246:1]())
    at LanguageClient.handleFailedRequest ([/Users/danny/Dev/Dart-Code/node_modules/vscode-languageclient/lib/common/client.js:2948:1]())
    at [/Users/danny/Dev/Dart-Code/node_modules/vscode-languageclient/lib/common/client.js:1917:1]()

If I put a breakpoint, it seems that the call to messageFunc is returning undefined but a Promise is assumed (it’s taking the error path):

(why showMessage is being called here with an error is another issue I need to investigate, but this feels like an issue on its own).

[dbaeumer]

Let me know what the outcome is. The messageFunc should point to existing VS Code API.

[DanTup]

Actually, this has always been this way - but before it was just an Information message and the test missed it. Most of the errors handled by this are actually prevented earlier by validation before we send the request, so I’ll just amend the test to handle it. There is no regression here (only a change in the severity of the error message shown by the LSP client).

(Sorry for another misfire 😬)