mojo: Allow circular dependencies between structs

mojo/python/tests/bindings_structs_unittest.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import math
 import unittest
 
 # pylint: disable=F0401
 import mojo.system
 
 # Generated files
 # pylint: disable=F0401
+import regression_tests_mojom
 import sample_import_mojom
 import sample_import2_mojom
 import sample_service_mojom
 
 
 class StructBindingsTest(unittest.TestCase):
 
   def testModule(self):
     self.assertEquals(sample_service_mojom.DefaultsTest.__module__,
                       'sample_service_mojom')
@@ skipping 176 matching lines @@
     p = sample_import_mojom.Point(34, y=12)
     self.assertEquals(p.x, 34)
     self.assertEquals(p.y, 12)
 
     with self.assertRaises(TypeError):
       p = sample_import_mojom.Point(0, 0, 0)
     with self.assertRaises(TypeError):
       p = sample_import_mojom.Point(0, x=0)
     with self.assertRaises(TypeError):
       p = sample_import_mojom.Point(c=0)
+
+  def testCyclicDefinition(self):
+    a = regression_tests_mojom.A()
+    b = regression_tests_mojom.B()
+    self.assertIsNone(a.b)
+    self.assertIsNone(b.a)
+    a.b = b
+    self.assertIs(a.b, b)