Ensure es3recast processes files one at a time.

Production builds before this change took approximately 70 seconds on my machine.
After this change they take ~52 seconds, and rebuilds take ~7 seconds.

__BEFORE__

```
Build successful - 70423ms.

Slowest Trees                  | Total
-------------------------------+----------------
ES3SafeFilter                  | 16020ms
ES3SafeFilter                  | 7443ms
ES3SafeFilter                  | 7278ms
ES3SafeFilter                  | 6979ms
DefeatureifyFilter             | 5292ms
ES3SafeFilter                  | 3916ms
ES3SafeFilter                  | 3615ms

file changed mixins/deferred_test.js

Build successful - 47688ms.
... Reload LiveReload files ...

Slowest Trees                  | Total
-------------------------------+----------------
ES3SafeFilter                  | 17385ms
ES3SafeFilter                  | 8278ms
ES3SafeFilter                  | 7518ms
ES3SafeFilter                  | 7363ms
ES3SafeFilter                  | 3658ms
```

__AFTER__

```
Build successful - 57506ms.

Slowest Trees                  | Total
-------------------------------+----------------
ES3SafeFilter                  | 16911ms
ES3SafeFilter                  | 9628ms
ES3SafeFilter                  | 9211ms
DefeatureifyFilter             | 6179ms

file changed main.js

Build successful - 6344ms.

Slowest Trees                  | Total
-------------------------------+----------------
UglifyJSFilter                 | 1252ms
```