monaco-editor version: 0.11.1
Browser: Chrome 64
OS: Windows 10
Steps or JS usage snippet reproducing the issue: Configure a develop environment using webpack and webpackDevServer. The dev server would post a message with type webpackOk (when the client-side dev server plugin is ready, I think).

    self.postMessage({
        type: 'webpack' + type,
        data: data
    }, '*');

In editor.worker.js, the worker received the message and passed the message to simpleWorker.js to handle.

    SimpleWorkerProtocol.prototype.handleMessage = function (serializedMessage) {  // got an object
        var message;
        try {
            message = JSON.parse(serializedMessage);
        }
        catch (e) {
            // nothing
        }
        if (!message.vsWorker) {
            return;
        }
        if (this._workerId !== -1 && message.vsWorker !== this._workerId) {
            return;
        }
        this._handleMessage(message);
    };

So in line 9 in simpleWorker.js, "Cannot read property 'vsWorker' of undefined" is thrown.

I think maybe we shall check if message === undefined first or simply ignore message with type prefix webpack?

If anyone thinks the webpack config file is necessary to provide more information, I would be happy to post it.