Invert coverage data from {test_context: {file: line}}
        to {file: {test_context: line}}

def _invert(self):
        """
        Invert coverage data from {test_context: {file: line}}
        to {file: {test_context: line}}
        """
        result = defaultdict(dict)
        for test_context, src_context in six.iteritems(self.data):
            for src, lines in six.iteritems(src_context):
                result[src][test_context] = lines
        return result