diff --git a/lib/legacy/class.js b/lib/legacy/class.js
index 08234ff41f..a625576cdc 100644
--- a/lib/legacy/class.js
+++ b/lib/legacy/class.js
@@ -10,10 +10,10 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
 
 import { LegacyElementMixin } from './legacy-element-mixin.js';
 import { legacyOptimizations, legacyNoObservedAttributes } from '../utils/settings.js';
+import { findObservedAttributesGetter } from '../mixins/disable-upgrade-mixin.js';
 import { wrap } from '../utils/wrap.js';
 
 const DISABLED_ATTR = 'disable-upgrade';
-let observedAttributesGetter;
 
 const lifecycleProps = {
   attached: true,
@@ -179,6 +179,9 @@ function mergeProperties(target, source) {
   }
 }
 
+const LegacyElement = LegacyElementMixin(HTMLElement);
+const observedAttributesGetter = findObservedAttributesGetter(LegacyElement);
+
 /* Note about construction and extension of legacy classes.
   [Changed in Q4 2018 to optimize performance.]
 
@@ -211,22 +214,6 @@ function mergeProperties(target, source) {
  */
 function GenerateClassFromInfo(info, Base, behaviors) {
 
-  // Work around for closure bug #126934458. Using `super` in a property
-  // getter does not work so instead we search the Base prototype for an
-  // implementation of observedAttributes so that we can override and call
-  // the `super` getter. Note, this is done one time ever because we assume
-  // that `Base` is always comes from `Polymer.LegacyElementMixn`.
-  if (!observedAttributesGetter) {
-    let ctor = Base;
-    while (ctor && !observedAttributesGetter) {
-      const desc = Object.getOwnPropertyDescriptor(ctor, 'observedAttributes');
-      if (desc) {
-        observedAttributesGetter = desc.get;
-      }
-      ctor = Object.getPrototypeOf(ctor.prototype).constructor;
-    }
-  }
-
   // manages behavior and lifecycle processing (filled in after class definition)
   let behaviorList;
   const lifecycle = {};
@@ -477,20 +464,20 @@ function GenerateClassFromInfo(info, Base, behaviors) {
       }
     }
 
-    // NOTE: Below is an inlined version of DisableUpgradeMixin. It is inlined
-    // as a performance optimization to avoid the need to place the mixin on
-    // top of every legacy element.
+    // NOTE: Inlined for perf from version of DisableUpgradeMixin.
     constructor() {
       super();
       /** @type {boolean|undefined} */
       this.__isUpgradeDisabled;
     }
 
+    // NOTE: Inlined for perf from version of DisableUpgradeMixin.
     static get observedAttributes() {
       return legacyNoObservedAttributes ? [] :
         observedAttributesGetter.call(this).concat(DISABLED_ATTR);
     }
 
+    // NOTE: Inlined for perf from version of DisableUpgradeMixin.
     // Prevent element from initializing properties when it's upgrade disabled.
     /** @override */
     _initializeProperties() {
@@ -501,6 +488,7 @@ function GenerateClassFromInfo(info, Base, behaviors) {
       }
     }
 
+    // NOTE: Inlined for perf from version of DisableUpgradeMixin.
     // Prevent element from enabling properties when it's upgrade disabled.
     // Normally overriding connectedCallback would be enough, but dom-* elements
     /** @override */
@@ -510,6 +498,7 @@ function GenerateClassFromInfo(info, Base, behaviors) {
       }
     }
 
+    // NOTE: Inlined for perf from version of DisableUpgradeMixin.
     // If the element starts upgrade-disabled and a property is set for
     // which an accessor exists, the default should not be applied.
     // This additional check is needed because defaults are applied via
@@ -521,6 +510,7 @@ function GenerateClassFromInfo(info, Base, behaviors) {
         !(this.__isUpgradeDisabled && this._isPropertyPending(property));
     }
 
+    // NOTE: Inlined for perf from version of DisableUpgradeMixin.
     /**
      * @override
      * @param {string} name Attribute name.
@@ -546,6 +536,7 @@ function GenerateClassFromInfo(info, Base, behaviors) {
       }
     }
 
+    // NOTE: Inlined for perf from version of DisableUpgradeMixin.
     // Prevent element from connecting when it's upgrade disabled.
     // This prevents user code in `attached` from being called.
     /** @override */
@@ -555,6 +546,7 @@ function GenerateClassFromInfo(info, Base, behaviors) {
       }
     }
 
+    // NOTE: Inlined for perf from version of DisableUpgradeMixin.
     // Prevent element from disconnecting when it's upgrade disabled.
     // This avoids allowing user code `detached` from being called without a
     // paired call to `attached`.
@@ -599,8 +591,6 @@ function GenerateClassFromInfo(info, Base, behaviors) {
   return PolymerGenerated;
 }
 
-const LegacyElement = LegacyElementMixin(HTMLElement);
-
 /**
  * Generates a class that extends `LegacyElement` based on the
  * provided info object.  Metadata objects on the `info` object
diff --git a/lib/mixins/disable-upgrade-mixin.js b/lib/mixins/disable-upgrade-mixin.js
index 868cfc49d2..f6d380fc2f 100644
--- a/lib/mixins/disable-upgrade-mixin.js
+++ b/lib/mixins/disable-upgrade-mixin.js
@@ -15,7 +15,16 @@ import { wrap } from '../utils/wrap.js';
 
 const DISABLED_ATTR = 'disable-upgrade';
 
-let observedAttributesGetter;
+export const findObservedAttributesGetter = (ctor) => {
+  while (ctor) {
+    const desc = Object.getOwnPropertyDescriptor(ctor, 'observedAttributes');
+    if (desc) {
+      return desc.get;
+    }
+    ctor = Object.getPrototypeOf(ctor.prototype).constructor;
+  }
+  return () => [];
+};
 
 /**
  * Element class mixin that allows the element to boot up in a non-enabled
@@ -58,16 +67,7 @@ export const DisableUpgradeMixin = dedupingMixin((base) => {
   // implementation of observedAttributes so that we can override and call
   // the `super` getter. Note, this is done one time ever because we assume
   // that `Base` is always comes from `Polymer.LegacyElementMixn`.
-  if (!observedAttributesGetter) {
-    let ctor = superClass;
-    while (ctor && !observedAttributesGetter) {
-      const desc = Object.getOwnPropertyDescriptor(ctor, 'observedAttributes');
-      if (desc) {
-        observedAttributesGetter = desc.get;
-      }
-      ctor = Object.getPrototypeOf(ctor.prototype).constructor;
-    }
-  }
+  let observedAttributesGetter = findObservedAttributesGetter(superClass);
 
   /**
    * @polymer
diff --git a/test/unit/legacy-noattributes.html b/test/unit/legacy-noattributes.html
index d44a347518..1594dd735b 100644
--- a/test/unit/legacy-noattributes.html
+++ b/test/unit/legacy-noattributes.html
@@ -32,7 +32,8 @@
         properties: {
           foo: {type: String, value: 'default'},
           bar: {type: String, value: 'default'},
-          zot: {type: Boolean, value: false, reflectToAttribute: true}
+          zot: {type: Boolean, value: false, reflectToAttribute: true},
+          camelCase: String
         },
         ready() {
           this.isEnabled = true;
@@ -43,12 +44,12 @@
 
   <dom-module id="x-attrs">
     <template>
-      <x-child id="child1" foo="x-attrs.foo" bar$="[[bar]]" zot$="[[zot]]"></x-child>
-      <x-child id="child2" foo="x-attrs.foo" bar$="[[bar]]" zot$="[[zot]]" disable-upgrade></x-child>
-      <x-child id="child3" foo="x-attrs.foo" bar$="[[bar]]" zot$="[[zot]]" disable-upgrade$="[[disabled]]"></x-child>
+      <x-child id="child1" foo="x-attrs.foo" bar$="[[bar]]" zot$="[[zot]]" camel-case="camelCase"></x-child>
+      <x-child id="child2" foo="x-attrs.foo" bar$="[[bar]]" zot$="[[zot]]" camel-case="[[camelCase]]" disable-upgrade></x-child>
+      <x-child id="child3" foo="x-attrs.foo" bar$="[[bar]]" zot$="[[zot]]" camel-case="[[camelCase]]" disable-upgrade$="[[disabled]]"></x-child>
       <div id="ifContainer">
         <dom-if if="[[shouldIf]]">
-          <template><x-child foo="x-attrs.foo" bar$="[[bar]]" zot$="[[zot]]"></x-child></template>
+          <template><x-child foo="x-attrs.foo" bar$="[[bar]]" zot$="[[zot]]" camel-case="[[camelCase]]"></x-child></template>
         </dom-if>
       </div>
     </template>
@@ -61,6 +62,7 @@
           bar: {type: String, value: 'bar'},
           zot: Boolean,
           shouldIf: Boolean,
+          camelCase: String,
           disabled: {type: Boolean, value: 'true'}
         }
       });
@@ -69,7 +71,7 @@
 
   <test-fixture id="declarative">
     <template>
-      <x-attrs id="configured" foo="foo" bar="bar"></x-attrs>
+      <x-attrs id="configured" foo="foo" bar="bar" camel-case="camelCase"></x-attrs>
     </template>
   </test-fixture>
 
@@ -89,6 +91,7 @@
         assert.equal(el.$.child1.getAttribute('foo'), 'x-attrs.foo');
         assert.equal(el.$.child1.foo, "x-attrs.foo");
         assert.equal(el.$.child1.$.content.textContent, 'x-attrs.foo');
+        assert.equal(el.camelCase, 'camelCase');
       });
 
       test('static attribute bindings', () => {
@@ -96,6 +99,7 @@
         assert.equal(el.$.child1.bar, 'bar');
         assert.equal(el.$.child1.getAttribute('zot'), null);
         assert.equal(el.$.child1.zot, false);
+        assert.equal(el.$.child1.camelCase, 'camelCase');
       });
 
       test('dynamic attribute bindings', () => {
@@ -114,6 +118,7 @@
         assert.equal(child.getAttribute('foo'), 'x-attrs.foo');
         assert.equal(child.foo, "x-attrs.foo");
         assert.equal(child.$.content.textContent, 'x-attrs.foo');
+        assert.equal(child.camelCase, 'camelCase');
         //
         assert.equal(child.getAttribute('bar'), 'bar');
         assert.equal(child.bar, 'bar');
@@ -132,12 +137,14 @@
         assert.notOk(el.$.child2.isEnabled);
         el.$.child2.removeAttribute('disable-upgrade');
         assert.isTrue(el.$.child2.isEnabled);
+        assert.equal(el.$.child2.camelCase, 'camelCase');
       });
 
       test('disable-upgrade binding', () => {
         assert.notOk(el.$.child3.isEnabled);
         el.disabled = false;
         assert.isTrue(el.$.child3.isEnabled);
+        assert.equal(el.$.child3.camelCase, 'camelCase');
       });