_expectParseError = function (cssCode, expectedCodeOffset, expectedErrorMessage) {
            try {
                _findMatchingRules(cssCode, null);

                // shouldn't get here since _findMatchingRules() is expected to throw
                this.fail("Expected parse error: " + cssCode);

            } catch (error) {
                expect(error.index).toBe(expectedCodeOffset);
                expect(error.message).toBe(expectedErrorMessage);
            }
        }
Test helper function: expects CSS parsing to fail at the given 0-based offset within the
cssCode string, with the given error message.