Address review feedback

* Uses `window.ShadyCSS.cssBuild` to avoid doing unnecessary work in `processElementStyles`
* Stores the list of active in-use behaviors in a legacy element's `behaviors` property. This only matters in the exotic case when elements with behaviors are extended with more behaviors and previously the list included only the subclasses behaviors (although the element otherwise worked as expected).

lib/legacy/class.html

@@ -203,9 +203,9 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
 
         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 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
 
         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 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
            `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 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
         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;

lib/mixins/element-mixin.html

@@ -21,6 +21,8 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
 (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 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
      * @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

test/unit/mixin-behaviors.html

@@ -551,7 +551,7 @@ suite('nested-behaviors element', function() {
   });
 
   test('nested-behavior dedups', function() {
-    assert.equal(el.behaviors.length, 2);
+    assert.equal(el.behaviors.length, 6);
   });
 
   test('nested-behavior lifecycle', function() {