Protractor - see text on page

I'm trying to write a test in Protractor, without highly coupling my test to specific markup of the page.

For example, given a typical log in page I'd want to test that if incorrect credentials are provided, an error message appears.

The way that my markup would display that error is:

<div class="alert alert-danger">
    <ul>
        <li>Invalid username or password.</li>
    </ul>
</div>

This unordered list may contain other errors, too - so I don't want to make an assertion on the list element itself. In any case, I may decide to move away from displaying the error as a list, and may want to show it in some other way.

All I want is to be able to assert that the page contains: Invalid username or password.

I'd like to be able to do something like:

expect(page.getContents()).to.contain('Invalid username or password.'); 

But of course this doesn't work.

Are there any methods available in Protractor that can allow me to do this?