Can't mock a property of an aliased class in Mockery

Question

I'm trying to test some code that calls the Google Drive API. Here's the code under test:

class GDrive {
    ...
    public function __construct(\Google_Client $client = null) {
        ...
    }
    ...
    $service = new \Google_Service_Drive($this->client);

    $root = $service->files->get($this->root, [
        "supportsAllDrives" => true
    ]);
    ...
}

$this->client is a mock of \Google_Client.

Here's the test:

$mock = Mockery::mock(Google_Client::class);
$this->gDrive = new GDrive($mock);

$gDriveMock = m::mock('overload:\Google_Service_Drive');
$gDriveFilesMock = m::mock('\Google\Service\Drive\Resource\Files');
$gDriveMock->files = $gDriveFilesMock;

I'm trying to mock out the call to $service->files->get, but when the $root = $service->files->get line is executed in GDrive I get:

Undefined property: Google_Service_Drive::$files

If I don't (attempt to) mock the classes out then the code works ok.

I guess there's a problem with either (or both):

• My use of overload to mock the directly instantiated Google_Service_Drive
• The -> syntax for setting a property on the mock

I've tried not using aliases in the references to the Google Drive API, but it doesn't appear to help.