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 0c16dbe5c76aa4d537dd3706c8430f3f10dfd8e9..0c1cef10783a8666d404050528e9893095dbc763 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;
 }
@@ -39255,6 +39257,11 @@ 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;