function openBlock(disableTracking = false) {
      blockStack.push((currentBlock = disableTracking ? null : []));
  }
Open a block.
This must be called before `createBlock`. It cannot be part of `createBlock`
because the children of the block are evaluated before `createBlock` itself
is called. The generated code typically looks like this:

```js
function render() {
  return (openBlock(),createBlock('div', null, [...]))
}
```
disableTracking is true when creating a v-for fragment block, since a v-for
fragment always diffs its children.

@private