Type casts get stripped by Babel JSX, alternate form needed

I'm preprocessing my JSX files with Babel, and it's stripping the extraneous comments needed only for Closure Compiler's type cast syntax, resulting in:

```
WARNING - Misplaced type annotation. Type annotations are not allowed here. Are you missing parentheses?
14| */
15| const x = function (a) {
16| let b = /** @type {string} */a;
17| b.indexOf(' ');};
18|
```

It would be good if there was an alternative syntax. Or if the type cast worked like Java's, and took effect on the immediate node following.

Research on whether there's a way to stop Babel doing this has been fruitless. And extending CC to do native JSX transpilation is probably very difficult. Would much appreciate any suggestions other than typing everything "?"!