webpack4 optimization.splitChunks的注意点

写了个共用的chunk，没有办法实现代码分离。
搞了一个多小时，最后发现官方文档的这句话，吐血了！

webpack will automatically split chunks based on these conditions:

New chunk can be shared OR modules are from the node_modules folder
New chunk would be bigger than 30kb (before min+gz)
Maximum number of parallel requests when loading chunks on demand would be lower or equal to 5
Maximum number of parallel requests at initial page load would be lower or equal to 3

New chunk would be bigger than 30kb (before min+gz)！
New chunk would be bigger than 30kb (before min+gz)！
New chunk would be bigger than 30kb (before min+gz)！

重要的事说三遍。

在chunk里故意写了一些无效代码，扩充到30Kb以后，终于分离成功。