I've been analyzing // +build usage in the Go ecosystem and turned up dom_wasmjs_gopherjs.go, which says:

// +build go1.12,wasm,js js

and dom_native.go which says:

// +build go1.12,!wasm,!js

These two conditions are:

dom_wasmjs_gopherjs.go: (go1.12 && wasm && js) || js which is a complex way of writing js.
dom_native.go: go1.12 && !wasm && !js

I am unsure about what exactly is intended here. From the comments in dom_native.go, it looks like the intent was for dom_wasmjs_gopherjs.go to apply to js/wasm builds and dom_native.go to apply otherwise. That would be:

dom_wasmjs_gopherjs.go: go1.12 && js && wasm

// +build go1.12,js,wasm

dom_native.go: !(go1.12 && js && wasm) which is !go1.12 || !js || !wasm

// +build !go1.12 !js !wasm

Best,
Russ