Hi

A module contains a top-level variable named toString

I can't call its exports that make use of this variable anymore

Works as expected on Ubuntu 16.10 with gjs 1.46.0-1

Broken on Ubuntu 17.04 with gjs 1.48.0-0ubuntu1

Steps to reproduce:

```
rm -rf ~/gjs-toString-regression
mkdir ~/gjs-toString-regression

cat <<EOF > ~/gjs-toString-regression/foo.js
var toString = x => x

this.default = function (x) {
  print(x, toString(x))
}
EOF

cat <<EOF > ~/gjs-toString-regression/test.js
const foo = imports.foo.default
foo('bar')
// Expected: bar bar
// Actual: bar [GjsModule foo]
EOF

cd ~/gjs-toString-regression
gjs -I `pwd` test.js
```

In this simplified example it can be fixed by renaming toString

But in real word toString is a complex import deep in an external dependency

Luckily my case is simple, using the module was just a matter of convenience

Someone will run into this in future though