Revert "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 5f9e117f0f2dcff73140e3feca7e060440e62c1f..dc8db31d9c58e5176022cbd72a00463d705c7494 100644
--- a/lib/html/dartium/html_dartium.dart
+++ b/lib/html/dartium/html_dartium.dart
@@ -9259,8 +9259,6 @@ class _ConsoleImpl extends NativeFieldWrapperClass1 implements Console {
 /// @domName HTMLContentElement
 abstract class ContentElement implements Element {
 
-  factory ContentElement() => _Elements.createContentElement();
-
   /** @domName HTMLContentElement.select */
   String select;
 }
@@ -42053,11 +42051,6 @@ 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;