"Unhandled declaration" in production


## *What* is wrong?
I get the following error when trying to use gpu.js on production (optimized code):
```console
Uncaught Error: Unhandled declaration
at WebGL2FunctionNode.astVariableDeclaration (gpu-browser.js:12600:17)
at WebGL2FunctionNode.astGeneric (gpu-browser.js:8023:23)
at WebGL2FunctionNode.astBlockStatement (gpu-browser.js:12555:14)
at WebGL2FunctionNode.astGeneric (gpu-browser.js:8007:23)
at WebGL2FunctionNode.astForStatement (gpu-browser.js:12455:12)
at WebGL2FunctionNode.astGeneric (gpu-browser.js:8017:23)
at WebGL2FunctionNode.astFunction (gpu-browser.js:11962:12)
at WebGL2FunctionNode.astFunctionExpression (gpu-browser.js:8095:17)
at WebGL2FunctionNode.astGeneric (gpu-browser.js:7991:23)
at WebGL2FunctionNode.toString (gpu-browser.js:7437:32)
```

## *Where* does it happen?
On [my](https://preview.mishasaidov.com/emojinator/) static website built with Next.js that loads gpu.js from CDN. However, it perfectly works on local server with unoptimized code in a dev mode.

## *How* do we replicate the issue?
1. Create Next.js project.
2. Add gpu.js script tag to the `_document.js`: ``
3. Build the project with `next build` (`npm run build`).
4. Generate static website with `next export`.
5. Host the static website on a server and try to use gpu.js there.

I suppose babel does something with my kernel code, idk.

## *How* important is this (1-5)?
4

## Other Comments
I use gpu.js v2.15.2. Here is my kernel code. Seems like nothing special:
```javascript
const createKernel = outputLength => new window.GPU({
mode: "gpu",
}).createKernel(
function (videoPixels, emojiPixels, emojiPixelsLength, brightness) {
let lowestDistanceIn3d = Infinity
let lowestDistanceEmojiPixelIndex = 0

for (let j = 0, i = 0; j < emojiPixelsLength; j += 3) {
const emojiPixel = [emojiPixels[j], emojiPixels[j + 1], emojiPixels[j + 2]]

const videoPixelOffset = this.thread.x * 4
const videoPixel = [
videoPixels[videoPixelOffset],
videoPixels[videoPixelOffset + 1],
videoPixels[videoPixelOffset + 2],
]

const distanceIn3d = Math.sqrt(
Math.pow(emojiPixel[0] - videoPixel[0] * brightness, 2) +
Math.pow(emojiPixel[1] - videoPixel[1] * brightness, 2) +
Math.pow(emojiPixel[2] - videoPixel[2] * brightness, 2)
)

if (lowestDistanceIn3d > distanceIn3d) {
lowestDistanceIn3d = distanceIn3d
lowestDistanceEmojiPixelIndex = i
}

i++
}
return lowestDistanceEmojiPixelIndex
},
{
output: [outputLength],
loopMaxIterations: 9999999,
}
)
```