Revert "Adding a mechanism to 'upgrade' the Dart type of elements"

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

Index: tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate b/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
index 3e44809eb5ab119c99c4fcb60c27b61190808dba..973c9d45e0219fd236128b8ce5343f2044138f7d 100644
--- a/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
+++ b/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
@@ -244,6 +244,32 @@ $endif
    * This custom element can also be instantiated via HTML using the syntax
    * `<input is="x-bar"></input>`
    *
+   * The [nativeTagName] parameter is needed by platforms without native support
+   * when subclassing a native type other than:
+   *
+   * * HtmlElement
+   * * SvgElement
+   * * AnchorElement
+   * * AudioElement
+   * * ButtonElement
+   * * CanvasElement
+   * * DivElement
+   * * ImageElement
+   * * InputElement
+   * * LIElement
+   * * LabelElement
+   * * MenuElement
+   * * MeterElement
+   * * OListElement
+   * * OptionElement
+   * * OutputElement
+   * * ParagraphElement
+   * * PreElement
+   * * ProgressElement
+   * * SelectElement
+   * * SpanElement
+   * * UListElement
+   * * VideoElement
    */
 $if DART2JS
   void register(String tag, Type customElementClass, {String extendsTag}) {
@@ -295,30 +321,4 @@ $endif
   @Experimental()
   Stream<Event> get onVisibilityChange =>
       visibilityChangeEvent.forTarget(this);
-
-  /// Creates an element upgrader which can be used to change the Dart wrapper
-  /// type for elements.
-  ///
-  /// The type specified must be a subclass of HtmlElement, when an element is
-  /// upgraded then the created constructor will be invoked on that element.
-  ///
-  /// If the type is not a direct subclass of HtmlElement then the extendsTag
-  /// parameter must be provided.
-  @Experimental()
-  ElementUpgrader createElementUpgrader(Type type, {String extendsTag}) {
-$if DART2JS
-    return new _JSElementUpgrader(this, type, extendsTag);
-$else
-    return new _VMElementUpgrader(this, type, extendsTag);
-$endif
-  }
-}
-
-/// A utility for changing the Dart wrapper type for elements.
-abstract class ElementUpgrader {
-  /// Upgrade the specified element to be of the Dart type this was created for.
-  ///
-  /// After upgrading the element passed in is invalid and the returned value
-  /// should be used instead.
-  Element upgrade(Element element);
 }