Apply LegacyDataMixin to TemplatizeInstanceBase. Fixes #5422

lib/legacy/legacy-data-mixin.js

@@ -11,6 +11,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
 import { Class } from './class.js';
 import { Polymer } from '../../polymer-legacy.js';
 import { dedupingMixin } from '../utils/mixin.js';
+import { templatize } from '../utils/templatize.js';
 
 const UndefinedArgumentError = class extends Error {
   constructor(message, arg) {
@@ -149,5 +150,14 @@ Polymer.Class = (info, mixin) => Class(info,
     LegacyDataMixin(superClass)
 );
 
+// Apply LegacyDataMixin to Templatizer instances as well, and defer
+// runtime switch to the root's host (_methodHost)
+templatize.mixin = 
+  dedupingMixin(superClass => class extends LegacyDataMixin(superClass) {
+    get _legacyUndefinedCheck() {
+      return this._methodHost && this._methodHost._legacyUndefinedCheck;
+    }
+  });
+
 console.info('LegacyDataMixin will be applied to all legacy elements.\n' +
               'Set `_legacyUndefinedCheck: true` on element class to enable.');

lib/utils/templatize.js

@@ -334,6 +334,10 @@ function createTemplatizerClass(template, templateInfo, options) {
   // Anonymous class created by the templatize
   let base = options.mutableData ?
     MutableTemplateInstanceBase : TemplateInstanceBase;
+    // Affordance for global mixins onto TemplatizeInstance
+    if (templatize.mixin) {
+      base = templatize.mixin(base);
+    }
   /**
    * @constructor
    * @extends {base}