Implement children for pure XBlocks in edx-platform

This requires fixing the javascript api implementation, and adding
an implementation of get_block to the ModuleSystem api.

However, the implementation is incomplete, due to mismatches between
the expectations of XModule and XBlock.

Also adds tests using the Acid block to make sure that the javascript
and python apis for children are working correctly.

common/lib/xmodule/xmodule/tests/__init__.py

@@ -78,6 +78,7 @@ def get_test_system(course_id=''):
         course_id=course_id,
         error_descriptor_class=ErrorDescriptor,
         get_user_role=Mock(is_staff=False),
+        descriptor_runtime=get_test_descriptor_system(),
     )
 
 

common/lib/xmodule/xmodule/x_module.py

@@ -1048,7 +1048,7 @@ class ModuleSystem(ConfigurableFragmentWrapper, Runtime):  # pylint: disable=abs
     """
     def __init__(
             self, static_url, track_function, get_module, render_template,
-            replace_urls, user=None, filestore=None,
+            replace_urls, descriptor_runtime, user=None, filestore=None,
             debug=False, hostname="", xqueue=None, publish=None, node_path="",
             anonymous_student_id='', course_id=None,
             open_ended_grading_interface=None, s3_interface=None,
@@ -1089,6 +1089,8 @@ class ModuleSystem(ConfigurableFragmentWrapper, Runtime):  # pylint: disable=abs
                          that capa_module can use to fix up the static urls in
                          ajax results.
 
+        descriptor_runtime - A `DescriptorSystem` to use for loading xblocks by id
+
         anonymous_student_id - Used for tracking modules with student id
 
         course_id - the course_id containing this module
@@ -1148,6 +1150,7 @@ class ModuleSystem(ConfigurableFragmentWrapper, Runtime):  # pylint: disable=abs
         self.get_real_user = get_real_user
 
         self.get_user_role = get_user_role
+        self.descriptor_runtime = descriptor_runtime
 
     def get(self, attr):
         """	provide uniform access to attributes (like etree)."""
@@ -1172,7 +1175,7 @@ class ModuleSystem(ConfigurableFragmentWrapper, Runtime):  # pylint: disable=abs
         return self.handler_url(self.xmodule_instance, 'xmodule_handler', '', '').rstrip('/?')
 
     def get_block(self, block_id):
-        raise NotImplementedError("XModules must use get_module to load other modules")
+        return self.get_module(self.descriptor_runtime.get_block(block_id))
 
     def resource_url(self, resource):
         raise NotImplementedError("edX Platform doesn't currently implement XBlock resource urls")