Adding constructor to ContentElement.

lib/html/dartium/html_dartium.dart

Index: lib/html/dartium/html_dartium.dart
diff --git a/lib/html/dartium/html_dartium.dart b/lib/html/dartium/html_dartium.dart
index 6bbd66146bbfa16491091bba494a7e3e364e96a9..482f106c1b9d2f4b5b19c35356ae2e969127b5dd 100644
--- a/lib/html/dartium/html_dartium.dart
+++ b/lib/html/dartium/html_dartium.dart
@@ -9259,6 +9259,8 @@ class _ConsoleImpl extends NativeFieldWrapperClass1 implements Console {
 /// @domName HTMLContentElement
 abstract class ContentElement implements Element {
 
+  factory ContentElement() => _Elements.createContentElement();
+
   /** @domName HTMLContentElement.select */
   String select;
 }
@@ -42050,6 +42052,11 @@ class _Elements {
     return _e;
   }
 
+  static ContentElement createContentElement() {
+    _HTMLContentElementImpl _e = _document.$dom_createElement("content");
+    return _e;
+  }
+
   static DListElement createDListElement() {
     _HTMLDListElementImpl _e = _document.$dom_createElement("dl");
     return _e;