Last Update: 12/3/2016
    VSS.getService(VSS.ServiceIds.Dialog).then(function(dialogService) {
        var extensionCtx = VSS.getExtensionContext();
        // Build absolute contribution ID for dialogContent
        var contributionId = extensionCtx.publisherId + "." + extensionCtx.extensionId + ".dialogContent";

        // Show dialog
        var dialogOptions = {
            title: "My Dialog Title",
            width: 800,
            height: 600
        };

        dialogService.openDialog(contributionId, dialogOptions);
    });

See Using host dialog for additional samples.