Making CallStack immutable, so it can be hashable

appengine/findit/crash/test/stacktrace_test_suite.py

 # Copyright 2016 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.
 
 from testing_utils import testing
 
 from crash.test.crash_testcase import CrashTestCase
 
 
 class StacktraceTestSuite(CrashTestCase):  #pragma: no cover.
 
   def _VerifyTwoStackFramesEqual(self, frame1, frame2):
+    self.assertIsNotNone(frame1, "the first frame is unexpectedly missing")
+    self.assertIsNotNone(frame2, "the second frame is unexpectedly missing")
     self.assertEqual(str(frame1), str(frame2))
     self.assertEqual(frame1.dep_path, frame2.dep_path)
 
   def _VerifyTwoCallStacksEqual(self, stack1, stack2):
-    self.assertEqual(len(stack1), len(stack2))
+    self.assertIsNotNone(stack1, "the first stack is unexpectedly missing")
+    self.assertIsNotNone(stack2, "the second stack is unexpectedly missing")
+    self.assertEqual(len(stack1.frames), len(stack2.frames))
     self.assertEqual(stack1.priority, stack2.priority)
     self.assertEqual(stack1.format_type, stack2.format_type)
     self.assertEqual(stack1.language_type, stack2.language_type)
-    map(self._VerifyTwoStackFramesEqual, stack1, stack2)
+    map(self._VerifyTwoStackFramesEqual, stack1.frames, stack2.frames)
 
-  def _VerifyTwoStacktracesEqual(self, stacktrace1, stacktrace2):
-    self.assertEqual(len(stacktrace1), len(stacktrace2))
-    map(self._VerifyTwoCallStacksEqual, stacktrace1, stacktrace2)
+  def _VerifyTwoStacktracesEqual(self, trace1, trace2):
+    self.assertIsNotNone(trace1, "the first trace is unexpectedly missing")
+    self.assertIsNotNone(trace2, "the second trace is unexpectedly missing")
+    self.assertEqual(len(trace1.stacks), len(trace2.stacks))
+    map(self._VerifyTwoCallStacksEqual, trace1.stacks, trace2.stacks)