* Fix path.popContext() to not try to load "-1" from contexts array.
The current implement of popContext does
```js
this.setContext(this.contexts[this.contexts.length - 1]);
```
even if `this.contexts` can be empty, which causes it to lookup the
property `"-1"`, which is not found on the array itself and obviously
also not in the `Object.prototype` and the `Array.prototype`. However
since `"-1"` is not a valid array index, but has a valid integer
representation, this is a very expensive lookup in V8 (and probably
other engines too, but that is probably less relevant, since Babel
most often runs on Node nowadays).
* Make zero check explicit (for readability).
Similar in spirit to https://github.com/babel/babel/pull/6580, the
current implementation did
```js
node.params[node.params.length - 1]
```
where `node.params` can also be empty, which causes it to lookup the
property `"-1"`, which is not found on the array itself and obviously
also not in the `Object.prototype` and the `Array.prototype`. However
since `"-1"` is not a valid array index, but has a valid integer
representation, this is a very expensive lookup in V8 (and probably
other engines too, but that is probably less relevant, since Babel
most often runs on Node nowadays). In V8 this causes a call to
the `%SetProperty` runtime function for each of these `"-1"`
property lookups, and in addition sends the whole `KeyedLoadIC`
to `MEGAMORPHIC` state, which also penalizes other accesses
on this line.
This is a small non-breaking performance fix.
Similar to the fixes in https://github.com/babel/babel/pull/6580 and
https://github.com/babel/babel/pull/6581, accesses of the form
```js
stack[stack.length - 1];
```
when `stack` can be an empty array are pretty bad for performance.
In this case it also breaks the type safety, since the function
`last<T>` is declared to only return values of type `T`, but
occasionally also returns `undefined` now, since the `stack` parameters
passed to it never contain a property `"-1"` and neither do the
`Object.prototype` or the `Array.prototype`.
This is a non-breaking performance fix, which adds proper checking
to ensure that `last` is only invoked on non-empty arrays.
