diff --git a/lib/legacy/class.html b/lib/legacy/class.html
index 671b297dd3..930015b757 100644
--- a/lib/legacy/class.html
+++ b/lib/legacy/class.html
@@ -203,9 +203,9 @@
 
         static get properties() {
           const properties = {};
-          if (this.prototype.behaviors) {
-            for (let i=0, b; i < this.prototype.behaviors.length; i++) {
-              b = this.prototype.behaviors[i];
+          if (this.prototype.__behaviors) {
+            for (let i=0, b; i < this.prototype.__behaviors.length; i++) {
+              b = this.prototype.__behaviors[i];
               mergeProperties(properties, b.properties);
             }
           }
@@ -215,9 +215,9 @@
 
         static get observers() {
           let observers = [];
-          if (this.prototype.behaviors) {
-            for (let i=0, b; i < this.prototype.behaviors.length; i++) {
-              b = this.prototype.behaviors[i];
+          if (this.prototype.__behaviors) {
+            for (let i=0, b; i < this.prototype.__behaviors.length; i++) {
+              b = this.prototype.__behaviors[i];
               if (b.observers) {
                 observers = observers.concat(b.observers);
               }
@@ -273,8 +273,8 @@
            `is` in `beforeRegister` as you could in 1.x.
           */
           const proto = this;
-          if (proto.hasOwnProperty('behaviors')) {
-            copyBehaviorProperties(proto.behaviors, proto.constructor);
+          if (proto.hasOwnProperty('__behaviors')) {
+            copyBehaviorProperties(proto.__behaviors, proto.constructor);
           }
           proto.__behaviorMetaProps = proto.__behaviorMetaProps || {};
           copyProperties(info, proto);
@@ -420,12 +420,12 @@
         if (!Array.isArray(behaviors)) {
           behaviors = [behaviors];
         }
-        let superBehaviors = PolymerGenerated.prototype.__allBehaviors;
+        let superBehaviors = PolymerGenerated.prototype.behaviors;
         // get flattened, deduped list of behaviors *not* already on super class
         behaviors = flattenBehaviors(behaviors, null, superBehaviors);
-        PolymerGenerated.prototype.__allBehaviors = superBehaviors ?
+        PolymerGenerated.prototype.behaviors = superBehaviors ?
           superBehaviors.concat(behaviors) : behaviors;
-        PolymerGenerated.prototype.behaviors = behaviors;
+        PolymerGenerated.prototype.__behaviors = behaviors;
       }
 
       PolymerGenerated.generatedFrom = info;
diff --git a/lib/mixins/element-mixin.html b/lib/mixins/element-mixin.html
index 3a0f3b7fad..dce111c712 100644
--- a/lib/mixins/element-mixin.html
+++ b/lib/mixins/element-mixin.html
@@ -21,6 +21,8 @@
 (function() {
   'use strict';
 
+  const builtCSS = window.ShadyCSS && window.ShadyCSS['cssBuild'];
+
   /**
    * Element class mixin that provides the core API for Polymer's meta-programming
    * features including template stamping, data-binding, attribute deserialization,
@@ -252,7 +254,7 @@
      * @private
      */
     function processElementStyles(klass, template, is, baseURI) {
-      if (!window.skipStyleIncludesAndUrls) {
+      if (!builtCSS) {
         const templateStyles = template.content.querySelectorAll('style');
         const stylesWithImports = Polymer.StyleGather.stylesFromTemplate(template);
         // insert styles from <link rel="import" type="css"> at the top of the template
diff --git a/test/unit/mixin-behaviors.html b/test/unit/mixin-behaviors.html
index 730f7ceb2b..6f4dc50428 100644
--- a/test/unit/mixin-behaviors.html
+++ b/test/unit/mixin-behaviors.html
@@ -551,7 +551,7 @@
   });
 
   test('nested-behavior dedups', function() {
-    assert.equal(el.behaviors.length, 2);
+    assert.equal(el.behaviors.length, 6);
   });
 
   test('nested-behavior lifecycle', function() {