diff --git a/externs/webcomponents-externs.js b/externs/webcomponents-externs.js
index 1d796ded48..ae141b4a90 100644
--- a/externs/webcomponents-externs.js
+++ b/externs/webcomponents-externs.js
@@ -66,3 +66,8 @@ HTMLTemplateElement.decorate = function(template){};
  * @param {function(function())} cb callback
  */
 CustomElementRegistry.prototype.polyfillWrapFlushCallback = function(cb){};
+
+/**
+ * @param {string} cssText
+ */
+CSSStyleSheet.prototype.replaceSync = function(cssText) {};
\ No newline at end of file
diff --git a/lib/legacy/class.js b/lib/legacy/class.js
index a625576cdc..967e635445 100644
--- a/lib/legacy/class.js
+++ b/lib/legacy/class.js
@@ -291,7 +291,7 @@ function GenerateClassFromInfo(info, Base, behaviors) {
         const a = this.attributes;
         for (let i=0, l=a.length; i < l; i++) {
           const attr = a[i];
-          this.__attributeReaction(attr.name, undefined, attr.value);
+          this.__attributeReaction(attr.name, null, attr.value);
         }
       }
       super.created();
@@ -309,6 +309,7 @@ function GenerateClassFromInfo(info, Base, behaviors) {
       }
     }
 
+    /** @override */
     setAttribute(name, value) {
       if (legacyNoObservedAttributes) {
         const oldValue = this.getAttribute(name);
@@ -319,6 +320,7 @@ function GenerateClassFromInfo(info, Base, behaviors) {
       }
     }
 
+    /** @override */
     removeAttribute(name) {
       if (legacyNoObservedAttributes) {
         const oldValue = this.getAttribute(name);
@@ -472,6 +474,7 @@ function GenerateClassFromInfo(info, Base, behaviors) {
     }
 
     // NOTE: Inlined for perf from version of DisableUpgradeMixin.
+    /** @override */
     static get observedAttributes() {
       return legacyNoObservedAttributes ? [] :
         observedAttributesGetter.call(this).concat(DISABLED_ATTR);
diff --git a/test/unit/legacy-noattributes.html b/test/unit/legacy-noattributes.html
index 1594dd735b..d07c91d5e0 100644
--- a/test/unit/legacy-noattributes.html
+++ b/test/unit/legacy-noattributes.html
@@ -64,6 +64,9 @@
           shouldIf: Boolean,
           camelCase: String,
           disabled: {type: Boolean, value: 'true'}
+        },
+        attributeChanged(name, old, value) {
+          this.attrInfo = {name, old, value};
         }
       });
     </script>
@@ -71,7 +74,13 @@
 
   <test-fixture id="declarative">
     <template>
-      <x-attrs id="configured" foo="foo" bar="bar" camel-case="camelCase"></x-attrs>
+      <x-attrs foo="foo" bar="bar" camel-case="camelCase"></x-attrs>
+    </template>
+  </test-fixture>
+
+  <test-fixture id="one-attr">
+    <template>
+      <x-attrs foo="foo"></x-attrs>
     </template>
   </test-fixture>
 
@@ -94,6 +103,21 @@
         assert.equal(el.camelCase, 'camelCase');
       });
 
+      test('attributeChanged gets expected arguments', () => {
+        el = fixture('one-attr');
+        assert.deepEqual(el.attrInfo, {name: 'foo', old: null, value: 'foo'});
+        el.setAttribute('zot', '');
+        assert.deepEqual(el.attrInfo, {name: 'zot', old: null, value: ''});
+        el.setAttribute('zot', 'foo');
+        assert.deepEqual(el.attrInfo, {name: 'zot', old: '', value: 'foo'});
+        el.removeAttribute('zot', 'foo');
+        assert.deepEqual(el.attrInfo, {name: 'zot', old: 'foo', value: null});
+        el.setAttribute('zot', 'bar');
+        assert.deepEqual(el.attrInfo, {name: 'zot', old: null, value: 'bar'});
+        el.setAttribute('foo', 'foo2');
+        assert.deepEqual(el.attrInfo, {name: 'foo', old: 'foo', value: 'foo2'});
+      })
+
       test('static attribute bindings', () => {
         assert.equal(el.$.child1.getAttribute('bar'), 'bar');
         assert.equal(el.$.child1.bar, 'bar');