Type assertion emit loses parenthesis

``` javascript
(x + 1 as number) * 3
```

drops the parens:

``` javascript
x + 1 * 3;
```

Playground link:
https://www.typescriptlang.org/play/#src=%0D%0A%20(x%20%2B%201%20as%20number)%20*%203%0D%0A%20
