prototype assignments create side-effects

Given this input:
```js
export function makeDecorator(name, parentClass) {
function DecoratorFactory(objOrType) {}
DecoratorFactory.prototype = Object.create(parentClass.prototype);
DecoratorFactory.prototype.toString = () => `@${name}`;
return DecoratorFactory;
}
function Parent() {}

const foo = makeDecorator('foo', Parent);
```

We should tree-shake away everything to 0, but we don't:
https://goo.gl/fqmTHt

Note that commenting out the two lines with `.prototype` does tree-shake.

/cc @mhevery