app.js

angular.module('myApp', ['myApp.services'])
.config(function(AppStateProvider) {
  AppStateProvider
    .setTesting(@@testing)
    .setProxyServer(@@proxyServer)
    .setBackendUrl(@@backendUrl)
    .setSocketUrl(@@socketUrl);
})

app_state.js

angular.module('myApp.services')
.provider('AppStateService', function() {
  var testingFlag   = false, 
      autoTestFlag  = false,
      proxyServer   = 'http://appapp.herokuapp.com/',
      backendUrl    = 'http://localhost:3000',
      socketUrl     = 'http://localhost:3000';

  this.setTesting = function(bool) { 
    if (typeof bool != 'undefined') {
      this.testingFlag = bool; 
    } else {
      this.testingFlag = true;
    }
    return this;
  }
  this.setProxyServer = function(server) {
    proxyServer = server || proxyServer;
    return this;
  }
  this.setBackendUrl = function(server) {
    backendUrl = server || backendUrl;
    return this;
  }
  this.setSocketUrl = function(server) {
    socketUrl = server || socketUrl;
    return this;
  }

  this.setAutoTest = function() { this.autoTestFlag = true; }

  this.$get = function() {
    var self = this;

    var service = {
      testMode: function() { return self.testingFlag; },
      autoTestingMode: function() { return service.testMode() && autoTestFlag; },
      proxyServer: function() { return proxyServer; },
      backendUrl: function() { return backendUrl; },
      socketUrl: function() { return socketUrl; }
    }

    return service;
  }
});

dev.json

{
  "proxyServer": "'http://appapp.herokuapp.com/'",
  "backendUrl": "'http://localhost:3000'",
  "socketUrl": "'http://localhost:3000'",
  "testing": true
}

Gruntfile.js

env = process.env.ENV || 'dev';
// ...
    replace: {
      dev: {
        options: {
          patterns: [{
            json: grunt.file.readJSON(__dirname + '/config/' + env + '.json'),
            app: '<%= yeoman.app %>/scripts/app.js'
          }]
        },
        files: [
          {
            expand: true, 
            cwd: '<%= yeoman.app %>',
            src: ['scripts/app.js'],
            dest: 'dev'
          }
        ]
      },
      dist: {
        options: {
          patterns: [{
            json: grunt.file.readJSON(__dirname + '/config/' + env + '.json')
          }]
        },
        files: [
          {
            expand: true, 
            cwd: '<%= yeoman.dist %>',
            src: ['scripts/scripts.js'],
            dest: '<%= yeoman.dist %>'
          }
        ]
      }
    },