mobx: @action decorator fail with typescript targeting ES3

The @action decorator seems to have no effect when compiled with typescript. Here is a codepen: https://codepen.io/testerez/pen/qNKVvO

The generated JS shows that typescript at least tries to apply the decorator:

    __decorate([
        action
    ], Data.prototype, "increment");

Switching to babel fixes the issue. Is that a typescript issue? I experience the same issue with typescript@beta (2.0)