How to make mocha test report in browser use Meteor sourcemap?

I use (practicalmeteor|dispatch):mocha as Meteor test driver. For server test they’re fine but when test fails in browser, they doesn’t use sourcemap to display the real filename:lineno in the test report. Am I missing something? Or is there any workaround?

As a dirty workaround, I add a ‘debugger’ statement to the assertion-error npm module. So that when any test failed in browser, it will stop there.