Function.prototype.toString error at render
Hello

_I am Reporting a bug or issue_

**Expected behavior:**
render to Sketch

**Observed behavior:**

Error is thrown when running npm run render

```
TypeError: undefined is not an object (evaluating 'Function.prototype.toString')
```
In all examples, the error is happening on this line

```
var funcToString = Function.prototype.toString;
```

which is part of this function

```
function isNative(fn) {
  // Based on isNative() from Lodash
  var funcToString = Function.prototype.toString;
  var hasOwnProperty = Object.prototype.hasOwnProperty;
  var reIsNative = RegExp('^' + funcToString
  // Take an example native function source for comparison
  .call(hasOwnProperty
  // Strip regex characters so we can use it for regex
  ).replace(/[\\^$.*+?()[\]{}|]/g, '\\$&'
  // Remove hasOwnProperty from the template to make it generic
  ).replace(/hasOwnProperty|(function).*?(?=\\\()| for .+?(?=\\\])/g, '$1.*?') + '$');
  try {
    var source = funcToString.call(fn);
    return reIsNative.test(source);
  } catch (err) {
    return false;
  }
}
```


**How to reproduce:**
Although I noticed the error on my personal project, it is also happening in the following example projects:

- Basic-setup
- Generative-colours
- styled-components
- profile-cards

**Sketch version:**
47 (45396)

**Node version**
tried on v8.7.0 and v9.3.0


---
I started getting this issue today, earlier in the week everything was running ok. I haven't made any knowing changes to my machine setup.

I was wondering if anyone else is having this issue or knows where to look further for debugging. Any help is greatly appreciated. 

