Adding constructor to ContentElement.

lib/html/dart2js/html_dart2js.dart

Index: lib/html/dart2js/html_dart2js.dart
diff --git a/lib/html/dart2js/html_dart2js.dart b/lib/html/dart2js/html_dart2js.dart
index c31e485b7ecc4188333da0fc783e55f15b660db2..3a49ffe226e9ada4161f557dc1999623ca71ee00 100644
--- a/lib/html/dart2js/html_dart2js.dart
+++ b/lib/html/dart2js/html_dart2js.dart
@@ -8401,6 +8401,8 @@ class _ConsoleImpl
 /// @domName HTMLContentElement
 abstract class ContentElement implements Element {
 
+  factory ContentElement() => _Elements.createContentElement();
+
   /** @domName HTMLContentElement.select */
   String select;
 }
@@ -39252,6 +39254,11 @@ class _Elements {
     return _e;
   }
 
+  static ContentElement createContentElement() {
+    _ContentElementImpl _e = _document.$dom_createElement("content");
+    return _e;
+  }
+
   static DListElement createDListElement() {
     _DListElementImpl _e = _document.$dom_createElement("dl");
     return _e;