Fix for scoping when class is not specified on element (null was prepended instead of empty string)

Author: nazar-pc

src/standard/x-styling.html

@@ -35,8 +35,8 @@
       },
 
       /**
-       * An element's style properties can be directly modified by 
-       * setting key-value pairs in `customStyle` on the element 
+       * An element's style properties can be directly modified by
+       * setting key-value pairs in `customStyle` on the element
        * (analogous to setting `style`) and then calling `updateStyles()`.
        *
        */
@@ -208,14 +208,14 @@
         }
         // note: using Polymer.dom here ensures that any attribute sets
         // will provoke distribution if necessary; do this iff necessary
-        node = (this.shadyRoot && this.shadyRoot._hasDistributed) ? 
+        node = (this.shadyRoot && this.shadyRoot._hasDistributed) ?
           Polymer.dom(node) : node;
         serializeValueToAttribute.call(this, value, attribute, node);
       },
 
       _scopeElementClass: function(element, selector) {
         if (!nativeShadow && !this._scopeCssViaAttr) {
-          selector += (selector ? ' ' : '') + SCOPE_NAME + ' ' + this.is +
+          selector = (selector ? selector + ' ' : '') + SCOPE_NAME + ' ' + this.is +
             (element._scopeSelector ? ' ' +  XSCOPE_NAME + ' ' +
             element._scopeSelector : '');
         }