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 @@
 
         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;

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

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() {