Making Stacktrace immutable, so it can be hashable

appengine/findit/crash/test/changelist_classifier_test.py

Index: appengine/findit/crash/test/changelist_classifier_test.py
diff --git a/appengine/findit/crash/test/changelist_classifier_test.py b/appengine/findit/crash/test/changelist_classifier_test.py
index ee802af61d228d0df83f9a719770d4c94eff2295..0caca65372c28d865aca2ce2f155b69cc736e199 100644
--- a/appengine/findit/crash/test/changelist_classifier_test.py
+++ b/appengine/findit/crash/test/changelist_classifier_test.py
@@ -107,6 +107,9 @@ DUMMY_CHANGELOG3 = ChangeLog.FromDict({
 # CrashReport directly and pulls out the revision_range and platform
 # itself; that way ChangelistClassifier.__call__ needn't worry about it,
 # allowing us to clean up the tests here.
+# TODO(wrengr): this empty stacktrace causes the unittests to emit
+# logging warnings about the fact that we can't get the crash_stack. Would
+# be nice to avoid that.
 DUMMY_REPORT = CrashReport(None, None, None, Stacktrace(), (None, None))
 
 class ChangelistClassifierTest(CrashTestSuite):