Using single-line arrow functions can cause closure compiler to inject an extra `return` keyword`.

It seems like adding braces here actual makes the final output smaller:

```
emscripten_console_log: (str) => { console.log(UTF8ToString(str)) }
```

Compared to:

```
emscripten_console_log: (str) => console.log(UTF8ToString(str))
```

It seems to happen when closure inlines `UTF8ToString` into `emscripten_console_log`. In the former case I supposed closure knows that the function returns undefined, but in the later case it adds an extra `return` keyword to return the result of the `console.log` call.

Maybe there is some way to teach closure that `console.log` always returns undefined so this is not needed?