Fix a bug in the dart lowlevel memcache implementation.

We assumed the order of the returned memcache items were the same as
the order given in the get request. This is not always the case. Specifically
the dev app server is not guaranteeing order.

This change makes the code do a quadratic lookup, which degenerates to linear
in the case the response is ordered.

BUG=
R=kustermann@google.com, sgjesse@google.com

Review URL: https://chromereviews.googleplex.com/115907016