diff --git a/polygerrit-ui/app/elements/gr-app.html b/polygerrit-ui/app/elements/gr-app.html
index d23de3de60..f49d8aaea0 100644
--- a/polygerrit-ui/app/elements/gr-app.html
+++ b/polygerrit-ui/app/elements/gr-app.html
@@ -27,7 +27,6 @@ limitations under the License.
 <link rel="import" href="/bower_components/polymer/polymer.html">
 <link rel="import" href="/bower_components/polymer-resin/standalone/polymer-resin.html">
 <!-- TODO(taoalpha): Remove once all legacyUndefinedCheck removed. -->
-<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({
diff --git a/polygerrit-ui/app/samples/bind-parameters.html b/polygerrit-ui/app/samples/bind-parameters.html
index a7eb39a542..a28c462bc0 100644
--- a/polygerrit-ui/app/samples/bind-parameters.html
+++ b/polygerrit-ui/app/samples/bind-parameters.html
@@ -15,7 +15,7 @@
   <script>
     Polymer({
       is: 'my-bind-sample',
-      _legacyUndefinedCheck: true,
+
       properties: {
         computedExample: {
           type: String,
diff --git a/polygerrit-ui/app/samples/repo-command.html b/polygerrit-ui/app/samples/repo-command.html
index 37aca04fc3..526d35051c 100644
--- a/polygerrit-ui/app/samples/repo-command.html
+++ b/polygerrit-ui/app/samples/repo-command.html
@@ -29,7 +29,7 @@
   <script>
     Polymer({
       is: 'repo-command-low',
-      _legacyUndefinedCheck: true,
+
       attached() {
         console.log(this.repoName);
         console.log(this.config);
diff --git a/polygerrit-ui/app/samples/some-screen.html b/polygerrit-ui/app/samples/some-screen.html
index 527ebcebbd..da025a20fd 100644
--- a/polygerrit-ui/app/samples/some-screen.html
+++ b/polygerrit-ui/app/samples/some-screen.html
@@ -38,7 +38,7 @@
   <script>
     Polymer({
       is: 'some-screen-main',
-      _legacyUndefinedCheck: true,
+
       properties: {
         rootUrl: String,
       },
diff --git a/polygerrit-ui/app/test/common-test-setup.html b/polygerrit-ui/app/test/common-test-setup.html
index 65c66b52fb..17dc05e3fe 100644
--- a/polygerrit-ui/app/test/common-test-setup.html
+++ b/polygerrit-ui/app/test/common-test-setup.html
@@ -62,6 +62,5 @@ limitations under the License.
 </script>
 <link rel="import"
     href="/bower_components/iron-test-helpers/iron-test-helpers.html" />
-<link rel="import" href="/bower_components/polymer/lib/legacy/legacy-data-mixin.html">
 <link rel="import" href="test-router.html" />
 <script src="/bower_components/moment/moment.js"></script>