Make invalid JS file fail Babel spec again

This change uses a different JavaScript syntax to cause the "when a .js file
does not start with 'use babel'" spec to fail again after upgrading to Electron
3 / Node 10.3.0.  The previous generator syntax seems to be supported in the
version of V8 used by Node 10 so we now need to use the 'export default' syntax
to trigger an error.

I've verified that changing the first line of this file to 'use babel' causes
the file to load successfully, so it seems this new syntax is effective to
verify the desired behavior.

spec/fixtures/babel/invalid.js

@@ -1,3 +1,3 @@
 'use 6to6';
 
-module.exports = async function* hello() {}
+export default 42;