Revert "Remove legacy-data-mixin"

This reverts commit 55f0ffa508fcde51405b385f2a44aeb321a07633.

Reason for revert: legacyUndefinedCheck is required for now

Change-Id: Id8eb5b632e46ed8301c48660f559bd35816836c8

polygerrit-ui/app/elements/gr-app-p2.html

diff --git a/polygerrit-ui/app/elements/gr-app-p2.html b/polygerrit-ui/app/elements/gr-app-p2.html
index 2fed982..dbac7fe 100644
--- a/polygerrit-ui/app/elements/gr-app-p2.html
+++ b/polygerrit-ui/app/elements/gr-app-p2.html
@@ -20,6 +20,7 @@
 
 <link rel="import" href="/bower_components/polymer/polymer.html">
 <link rel="import" href="/bower_components/polymer-resin/standalone/polymer-resin.html">
+<link rel="import" href="/bower_components/polymer/lib/legacy/legacy-data-mixin.html">
 <link rel="import" href="../behaviors/safe-types-behavior/safe-types-behavior.html">
 <script>
   security.polymer_resin.install({