You can clone with
Stack trace collection requires Error.collectStackTrace, which is a V8 extension.
For other browsers, can we maybe throw an Error, immediately catch it, and copy over the stack property?
The "Assertion.includeStack is true" test in the Online Test Suite is failing in Safari 6.0.2 on OS X 10.8.2 too. Safari does not use V8 either.
Bump this, it can be quite debugging friendly with stack trace instead of just the assertion error.