`StubArray` workaround is incompatible with terser's `unsafe_arrows` option

[Brad Zacher]

Intended outcome:

MobX should work with terser's unsafe_arrows optimisation option.

Actual outcome:

TypeError: Object.setPrototypeOf called on null or undefined

Info

mobx/packages/mobx/src/types/legacyobservablearray.ts

Lines 38 to 49 in 58d4328

 // Typescript workaround to make sure ObservableArray extends Array 
 class StubArray {} 
 function inherit(ctor, proto) { 
     if (Object.setPrototypeOf) { 
         Object.setPrototypeOf(ctor.prototype, proto) 
     } else if (ctor.prototype.__proto__ !== undefined) { 
         ctor.prototype.__proto__ = proto 
     } else { 
         ctor.prototype = proto 
     } 
 } 
 inherit(StubArray, Array.prototype) 

The mobx codebases utilises an empty class here.
This empty class is downlevelled by mobx's build process to be var StubArray = function StubArray() {}; (unpkg).

When terser runs with the [unsafe_arrows](https://terser.org/docs/options/#:~:text=unsafe_arrows%20(default%3A,2015%20or%20greater.) option it will convert all functions that don't reference this to an arrow, converting the above line to var StubArray = () => {}. This then breaks the inherit function which assumes StubArray.proto is defined.

Note: unsafe_arrows is (by definition) an unsafe optimisation - though in my experience it is generally safe because it's so rare to do operations on a function's prototype (outside of old-school "class" code).

It would be great if this line could be changed to make mobx more minifyable.