function setBlockTracking(value) {
      isBlockTreeEnabled += value;
  }
Block tracking sometimes needs to be disabled, for example during the
creation of a tree that needs to be cached by v-once. The compiler generates
code like this:

``` js
_cache[1] || (
  setBlockTracking(-1),
  _cache[1] = createVNode(...),
  setBlockTracking(1),
  _cache[1]
)
```

@private