Tests display "Error: Namespace "ol.webgl" already declared."

Running tests gives an error message:
```
Error: Namespace "ol.webgl" already declared.

  http://localhost:9810/input/test/closure/goog/base.js:195
  http://localhost:9810/input/test/$$/src/ol/webgl/webgl.js:1
```

`git bisect` identified the error being introduced with commit 393566155ded01057f924514d7738fd015ee96c7.

I used the following script to identify the error:
```bash
#!/bin/bash
./build.py test | grep -q Namespace
if [ $? == 0 ] ; then
	echo error present
	exit 1
else
	echo error not present
	exit 0
fi
```

You can run the bisection with:
```
git bisect start master f2ce4db
git bisect run ./test.sh
```

@fredj, maybe one for you?