Ensure telemetry system works with legacyNoObservedAttributes setting

lib/legacy/legacy-element-mixin.js

@@ -23,6 +23,7 @@ import { wrap } from '../utils/wrap.js';
 import { scopeSubtree } from '../utils/scope-subtree.js';
 import { legacyOptimizations, legacyNoObservedAttributes } from '../utils/settings.js';
 import { findObservedAttributesGetter } from '../mixins/disable-upgrade-mixin.js';
+import { register } from '../utils/telemetry.js';
 
 const DISABLED_ATTR = 'disable-upgrade';
 
@@ -165,8 +166,16 @@ export const LegacyElementMixin = dedupingMixin((base) => {
 
     // NOTE: Inlined for perf from version of DisableUpgradeMixin.
     static get observedAttributes() {
-      return legacyNoObservedAttributes ? [] :
-        observedAttributesGetter.call(this).concat(DISABLED_ATTR);
+      if (legacyNoObservedAttributes) {
+        // Ensure this element is property registered with the telemetry system.
+        if (!this.hasOwnProperty(JSCompiler_renameProperty('__observedAttributes', this))) {
+          this.__observedAttributes = [];
+          register(this.prototype);
+        }
+        return this.__observedAttributes;
+      } else {
+        return observedAttributesGetter.call(this).concat(DISABLED_ATTR);
+      }
     }
 
     // NOTE: Inlined for perf from version of DisableUpgradeMixin.

test/unit/legacy-noattributes.html

@@ -92,6 +92,7 @@
   <script type="module">
     import {flush} from '../../lib/utils/flush.js';
     import {wrap} from '../../lib/utils/wrap.js';
+    import {registrations} from '../../lib/utils/telemetry.js';
 
     suite('legacyNoObservedAttributes', () => {
 
@@ -100,6 +101,11 @@
         el = fixture('declarative');
       });
 
+      test('telemetry collected as expected', () => {
+        const def = registrations.find((def) => def.is === el.localName);
+        assert.ok(def);
+      });
+
       test('static attributes', () => {
         assert.equal(el.foo, 'foo');
         assert.equal(el.$.child1.getAttribute('foo'), 'x-attrs.foo');