Request: option for refinement by console.assert

Hello,

would it be feasable to add an option giving `console.assert` the same semantics as `throw`-statements with regard to [refinements](https://flow.org/en/docs/lang/refinements/#refinement-invalidations-)? The below example currently points out that `.parentNode` may be `undefined`:
```js
function remove(node: Node): Node {
return node.parentNode.removeChild(node)
}
```
Inserting an if-throw/-return statement refines the type of `.parentNode` for the else-branch. The type-checker is happy with this:
```js
function remove(node: Node): Node {
if (node.parentNode == null) throw new Error('cannot remove orphaned node')
return node.parentNode.removeChild(node)
}
```
Although `console.assert` does not change execution flow, the intend is otherwise the same. In addition, console-statements can be stripped once the tests pass.
```js
function remove(node: Node): Node {
console.assert(node.parentNode, 'cannot remove orphaned node')
return node.parentNode.removeChild(node)
}
```
This of course assumes, the application is not supposed to recover from such an error.