The condition only applies in unicode mode, where any lone surrogates
are desugared into a character class (and will not be considered in this
optimization). Non-unicode mode treats lone surrogates exactly like
any other codepoint.

BUG=chromium:711092

Review-Url: https://codereview.chromium.org/2808403006
Cr-Commit-Position: refs/heads/master@{#44638}