Add dart to JSON schema. (#5218)

* Add dart to JSON schema.

* Use spaces instead of tab.

tests/python/test_basic_models.py

@@ -13,13 +13,13 @@ dtest = xgb.DMatrix(dpath + 'agaricus.txt.test')
 rng = np.random.RandomState(1994)
 
 
-def json_model(model_path):
+def json_model(model_path, parameters):
     X = np.random.random((10, 3))
     y = np.random.randint(2, size=(10,))
 
     dm1 = xgb.DMatrix(X, y)
 
-    bst = xgb.train({'tree_method': 'hist'}, dm1)
+    bst = xgb.train(parameters, dm1)
     bst.save_model(model_path)
 
     with open(model_path, 'r') as fd:
@@ -285,7 +285,8 @@ class TestModels(unittest.TestCase):
 
     def test_model_json_io(self):
         model_path = './model.json'
-        j_model = json_model(model_path)
+        parameters = {'tree_method': 'hist', 'booster': 'gbtree'}
+        j_model = json_model(model_path, parameters)
         assert isinstance(j_model['learner'], dict)
 
         bst = xgb.Booster(model_file='./model.json')
@@ -306,5 +307,12 @@ class TestModels(unittest.TestCase):
         doc = os.path.join(path, 'doc', 'model.schema')
         with open(doc, 'r') as fd:
             schema = json.load(fd)
-        jsonschema.validate(instance=json_model(model_path), schema=schema)
+        parameters = {'tree_method': 'hist', 'booster': 'gbtree'}
+        jsonschema.validate(instance=json_model(model_path, parameters),
+                            schema=schema)
+        os.remove(model_path)
+
+        parameters = {'tree_method': 'hist', 'booster': 'dart'}
+        jsonschema.validate(instance=json_model(model_path, parameters),
+                            schema=schema)
         os.remove(model_path)