Edit in JSFiddle

var myApp = angular.module('myApp', []);

myApp.controller('fooCtrl', ['$scope', '$injector', function ($scope, $injector) {

    $scope.serviceName = 'barService';

    $scope.youAre = function () {
        $injector.get($scope.serviceName).myNameIs();
    };
}]);

myApp.service('barService', [function () {
    this.myNameIs = function () {
        alert('barService');
    };
}]);

myApp.service('bazService', [function () {
    this.myNameIs = function () {
        alert('bazService');
    };
}]);

<div ng-app="myApp" ng-controller="fooCtrl">
    <select ng-model="serviceName">
        <option value="barService">barService</option>
        <option value="bazService">bazService</option>
    </select>
    <button ng-click="youAre()">You are ...</button>
</div>