Remove assertIsModel and isDataSource

Use `instanceof` operator instead:

  ModelCtor.prototype instanceof loopback.Model
  dataSource instanceof loopback.DataSource

lib/application.js

@@ -141,7 +141,8 @@ app.model = function (Model, config) {
     configureModel(Model, config, this);
     isPublic = config.public !== false;
   } else {
-    assertIsModel(Model);
+    assert(Model.prototype instanceof loopback.Model,
+      'Model must be a descendant of loopback.Model');
   }
 
   var modelName = Model.modelName;
@@ -163,14 +164,6 @@ app.model = function (Model, config) {
   return Model;
 };
 
-function assertIsModel(Model) {
-  assert(typeof Model === 'function',
-    'Model must be a function / constructor');
-  assert(Model.modelName, 'Model must have a "modelName" property');
-  assert(Model.prototype instanceof loopback.Model,
-    'Model must be a descendant of loopback.Model');
-}
-
 /**
  * Get the models exported by the app. Returns only models defined using `app.model()`
  *
@@ -605,7 +598,8 @@ function dataSourcesFromConfig(config, connectorRegistry) {
 }
 
 function configureModel(ModelCtor, config, app) {
-  assertIsModel(ModelCtor);
+  assert(ModelCtor.prototype instanceof loopback.Model,
+    'Model must be a descendant of loopback.Model');
 
   var dataSource = config.dataSource;
 
@@ -613,7 +607,7 @@ function configureModel(ModelCtor, config, app) {
     dataSource = app.dataSources[dataSource];
   }
 
-  assert(isDataSource(dataSource),
+  assert(dataSource instanceof DataSource,
       ModelCtor.modelName + ' is referencing a dataSource that does not exist: "' +
         config.dataSource +'"');
 
@@ -692,10 +686,6 @@ function tryReadDir() {
   }
 }
 
-function isDataSource(obj) {
-  return obj instanceof DataSource;
-}
-
 function tryReadConfig(cwd, fileName) {
   try {
     return require(path.join(cwd, fileName + '.json'));