gistfile1.js

angular.module('foo', [])
.config(function($provide) {
  $provide.decorator('$rootScope', function ($delegate) {
    // Shim needed for browsers I don't care about
    var proto = Object.getPrototypeOf($delegate);

    proto.$bar = function() {
      // Do something
    };

    return $delegate;
  })
})
.controller('MainCtrl', function($scope) {
  $scope.$bar();
});