pushd and popd are considered errors

The following script
```fish
pushd foo
popd
```

will cause errors when validated with syntastic or the supplied fish compiler:
```
:!fish --no-execute /tmp/bug.fish  2>&1| tee /tmp/nvimVvLAs5/100
/tmp/bug.fish (line 1):
pushd foo
^
/tmp/bug.fish (line 2):
popd
^
```