If a comparer isn't provided, it was silently assuming the element type
implements Comparable, even though that isn't required. Fixed to add a
manual cast at each comparison.
A stricter faster approach might be to check that the type argument
itself is a type that implements Comparable, but I don't know if that's
too pessimistic, or even possible to do.
R=vsm@google.com
Review URL: https://codereview.chromium.org/2311513002 .
Bob Nystrom
33ff9a3d4f