Change `@proxy()` to `@proxy`

https://github.com/angular/angular/commit/3c58878b19b015b14d2d223a063efc25ea7f1419#diff-a7b00ed9077569881da8d1f71374e836 caused a regression in Dart output. `@proxy()` is an incorrect way to use `proxy`. It must be `@proxy` as it is a predefined constant.

/cc @jelbourn 
