automatically convert <shadow></shadow> to <shadow><content></content…

…></shadow>

See spec bug for discussion: https://www.w3.org/Bugs/Public/show_bug.cgi?id=22460

src/declaration/prototype.js

@@ -68,6 +68,9 @@
       this.parseHostEvents();
       // install external stylesheets as if they are inline
       this.installSheets();
+      //
+      this.adjustShadowElement();
+      //
       // TODO(sorvell): install a helper method this.resolvePath to aid in 
       // setting resource paths. e.g.
       // this.$.image.src = this.resolvePath('images/foo.png')
@@ -83,6 +86,20 @@
         this.prototype.registerCallback(this);
       }
     },
+    // TODO(sorvell): remove when spec addressed:
+    // https://www.w3.org/Bugs/Public/show_bug.cgi?id=22460
+    // make <shadow></shadow> be <shadow><content></content></shadow>
+    adjustShadowElement: function() {
+      var content = this.templateContent();
+      if (content) {
+        var s$ = content.querySelectorAll('shadow');
+        for (var i=0, l=s$.length, s; (i<l) && (s=s$[i]); i++) {
+          if (!s.children.length) {
+            s.appendChild(document.createElement('content'));
+          }
+        }
+      }
+    },
     // if a named constructor is requested in element, map a reference
     // to the constructor to the given symbol
     publishConstructor: function() {