Fixed #25466 -- Added backwards compatibility aliases for LoaderOrigin and StringOrigin.

Thanks Simon Charette for the DeprecationInstanceCheck class.

django/template/__init__.py

@@ -58,7 +58,8 @@ from .exceptions import TemplateDoesNotExist, TemplateSyntaxError       # NOQA i
 
 # Template parts
 from .base import (                                                     # NOQA isort:skip
-    Context, Node, NodeList, Origin, RequestContext, Template, Variable,
+    Context, Node, NodeList, Origin, RequestContext, StringOrigin, Template,
+    Variable,
 )
 
 # Library management

django/template/base.py

@@ -59,6 +59,9 @@ from django.template.context import (  # NOQA: imported for backwards compatibil
     BaseContext, Context, ContextPopException, RequestContext,
 )
 from django.utils import six
+from django.utils.deprecation import (
+    DeprecationInstanceCheck, RemovedInDjango20Warning,
+)
 from django.utils.encoding import (
     force_str, force_text, python_2_unicode_compatible,
 )
@@ -156,6 +159,11 @@ class Origin(object):
             )
 
 
+class StringOrigin(six.with_metaclass(DeprecationInstanceCheck, Origin)):
+    alternative = 'django.template.Origin'
+    deprecation_warning = RemovedInDjango20Warning
+
+
 class Template(object):
     def __init__(self, template_string, origin=None, name=None, engine=None):
         try:

django/template/loader.py

@@ -1,4 +1,10 @@
+from django.utils import six
+from django.utils.deprecation import (
+    DeprecationInstanceCheck, RemovedInDjango20Warning,
+)
+
 from . import engines
+from .base import Origin
 from .exceptions import TemplateDoesNotExist
 
 
@@ -57,3 +63,8 @@ def render_to_string(template_name, context=None, request=None, using=None):
 
 def _engine_list(using=None):
     return engines.all() if using is None else [engines[using]]
+
+
+class LoaderOrigin(six.with_metaclass(DeprecationInstanceCheck, Origin)):
+    alternative = 'django.template.Origin'
+    deprecation_warning = RemovedInDjango20Warning