Array check failed on Array subclasses

```js
const MyArr = class extends Array {}
const foo = new MyArr()
Array.isArray(foo) // should be true
```
but with buble it actually shows `false`, while babel does this right: [link](https://babeljs.io/repl#?babili=false&browsers=&build=&builtIns=false&spec=false&loose=false&code_lz=MYewdgzgLgBAsgTwIICcUwLw2AGwIYQQwCmAHlMWACZGop4IDeAvgFCiSwBmIImMYYgHd4yNAAoAlO3AQQOYgDocIAObi6DRQEsImhOJ4hJkoA&debug=false&forceAllTransforms=false&shippedProposals=false&circleciRepo=&evaluate=false&fileSize=false&timeTravel=false&sourceType=module&lineWrap=true&presets=es2015%2Creact%2Cstage-2&prettier=false&targets=&version=7.4.5&externalPlugins=).

TypeScript also does this right: [link](https://www.typescriptlang.org/play/?target=1#code/MYewdgzgLgBAsgTwIICcUwLw2AGwIYQQwCmAHlMWACZGop4IDeAvgLABQoksAZiCJhhhiAd3jI0ACgCUHLhBA5iAOhwgA5pLoNlASwjaEkviGnSgA)

Should this be fixed as a bug?