According to the How do I assert an element is focused? thread, you can check if an element is focused by switching to an activeElement() and assert this is the
It's weird that it's expecting a promise only and could not handle a webdriver element... I had the same HUGE stacktrace as you.
Anyway, would you accept this kind of solution: send a "dumb" promise with a nice comment to justify why you had to do that. It's more a workaround than a semantic solution I admit.
expect(page.element.getInnerHtml())
.toEqual(browser.driver.switchTo().activeElement().getInnerHtml());
It's working for me ;)
EDIT: Bonus answer
The reason you can't call expect with a WebElement comes from the Webdriver Control Flow Principle (I'm sure you already know about) and this line in jasminewd, the adapter for jasmine to Webdriver developped and used by Protractor ;)