Polymer 2: Add _legacyUndefinedCheck

When legacy-data-mixin.html is imported this will tell you on the console which observers need fixing regarding this change of behavior:

https://polymer-library.polymer-project.org/2.0/docs/upgrade#update-observers

This change was created by running this command line:

find polygerrit-ui/app/ -type f | xargs sed -i '/^    is: / a\    _legacyUndefinedCheck: true,'

Once all observers are fixed, this change will be reverted. This is tracked in https://bugs.chromium.org/p/gerrit/issues/detail?id=10723

Change-Id: Icccdfaa52bf77dfb25e949b2aa2402110f6c2276

polygerrit-ui/app/elements/settings/gr-change-table-editor/gr-change-table-editor.js

diff --git a/polygerrit-ui/app/elements/settings/gr-change-table-editor/gr-change-table-editor.js b/polygerrit-ui/app/elements/settings/gr-change-table-editor/gr-change-table-editor.js
index 7d109633..01cc6df 100644
--- a/polygerrit-ui/app/elements/settings/gr-change-table-editor/gr-change-table-editor.js
+++ b/polygerrit-ui/app/elements/settings/gr-change-table-editor/gr-change-table-editor.js
@@ -19,6 +19,7 @@
 
   Polymer({
     is: 'gr-change-table-editor',
+    _legacyUndefinedCheck: true,
 
     properties: {
       displayedColumns: {