use ReactiveFlags instead of __v_ properties

[shengxinjing]

No description provided.

[yyx990803]

I initially avoided doing this because TS transpiles it to bracket notation (foo["bar"]). But then I did some benchmarking and it seems it doesn't really make a difference, and in production minifiers like terser will convert this to dot notation anyway.

[shengxinjing]

I realize it，thanks for reply