Revert "Revert "Start of dartium custom element lifecycle event support.""

tools/dom/templates/html/impl/impl_Document.darttemplate

Index: tools/dom/templates/html/impl/impl_Document.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Document.darttemplate b/tools/dom/templates/html/impl/impl_Document.darttemplate
index 90c3dfa6b00752da11cdb4e0418a97e4e55e38a5..7cc7f09e64e32251e884536000fd1dddb7f12b2f 100644
--- a/tools/dom/templates/html/impl/impl_Document.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Document.darttemplate
@@ -36,4 +36,18 @@ $else
     return true;
 $endif
   }
+
+  @DomName('Document.createElement')
+  Element createElement(String tagName, [String typeExtension]) {
+$if DART2JS
+    return _createElement(tagName, typeExtension);
+$else
+    if (typeExtension != null) {
+      return _createElement(tagName, typeExtension);
+    } else {
+      // Fast-path for Dartium when typeExtension is not needed.
+      return _Utils.createElement(this, tagName);
+    }
+$endif
+  }
 }