Oh well, I found it, just attach to the $window
and you are done.
As written here: https://developers.braintreepayments.com/javascript+node/sdk/client/setup
"The SDK will appear as braintree on the global window object."
So from your controller (for example) you can simply use $window.braintree
and you got everything you need from Braintree client library.
To load the Dropin you can simply use this:
angular.module('app').controller('YourController', ['$scope', '$window',
function ($scope, $window) {
$window.braintree.setup('CLIENTTOKEN', 'dropin', {
container: 'dropin'
});
}
]);