Auto-load and register built-in `Checkpoint` model

common/models/change.js

@@ -441,7 +441,7 @@ Change.rectifyAll = function(cb) {
 Change.getCheckpointModel = function() {
   var checkpointModel = this.Checkpoint;
   if(checkpointModel) return checkpointModel;
-  this.checkpoint = checkpointModel = require('./checkpoint').extend('checkpoint');
+  this.checkpoint = checkpointModel = loopback.Checkpoint.extend('checkpoint');
   assert(this.dataSource, 'Cannot getCheckpointModel(): ' + this.modelName
     + ' is not attached to a dataSource');
   checkpointModel.attachTo(this.dataSource);

lib/builtin-models.js

@@ -8,6 +8,7 @@ module.exports = function(loopback) {
   loopback.ACL = require('../common/models/acl').ACL;
   loopback.Scope = require('../common/models/acl').Scope;
   loopback.Change = require('../common/models/change');
+  loopback.Checkpoint = require('../common/models/checkpoint');
 
   /*!
    * Automatically attach these models to dataSources

test/checkpoint.test.js

@@ -2,7 +2,7 @@ var async = require('async');
 var loopback = require('../');
 
 // create a unique Checkpoint model
-var Checkpoint = require('../common/models/checkpoint').extend('TestCheckpoint');
+var Checkpoint = loopback.Checkpoint.extend('TestCheckpoint');
 Checkpoint.attachTo(loopback.memory());
 
 describe('Checkpoint', function() {