Power BI Custom visual testing ES6

Hello there,

I am trying to write test for my custom visual by following guide on https://microsoft.github.io/PowerBI-visuals/docs/how-to-guide/unit-tests-introduction/

but I am facing following error

Chrome 71.0.3578 (Windows 10.0.0) ERROR
    Uncaught SyntaxError: Unexpected token {
    at node_modules/powerbi-visuals-utils-testutils/lib/index.js:1
Chrome 71.0.3578 (Windows 10.0.0) ERROR
    Uncaught SyntaxError: Unexpected token {
    at node_modules/powerbi-visuals-utils-testutils/lib/index.js:1

I am using PBIVIZ v3.x.x to develop custom visual.
How do I resolve this error? Where can I find updated guide on testing custom visual using new pbiviz tools?