6 years late I know… But I’m hitting the same problem. I don’t think it’s solved yet.
I solved it manually by adding the missing class, and that is not hard at all. I made a file WebGL2RenderingContext.kt which contains just a copy of the WebGLRenderingContext class, with just the class name changed to add the ‘2’. This has been working well for me, though I haven’t been using anything in Kotlin using this, only used it for the newer GLSL language.
If you need methods on the WebGL2 object that don’t exist on the WebGL1 I expect you can simply declare them so they get bridged to the javascript version.