Re-apply "Adding a mechanism to 'upgrade' the Dart type of elements"

tools/dom/src/shared_html.dart

Index: tools/dom/src/shared_html.dart
diff --git a/tools/dom/src/shared_html.dart b/tools/dom/src/shared_html.dart
index 7c4f6960ad009f2bb7ffd1f4ec3e86b2a44a0ac8..dd67c48a87d1918ffc71277f06ea8c80d7717135 100644
--- a/tools/dom/src/shared_html.dart
+++ b/tools/dom/src/shared_html.dart
@@ -68,3 +68,12 @@ Element querySelector(String selectors) => document.querySelector(selectors);
  * [CSS selector specification](http://www.w3.org/TR/css3-selectors/).
  */
 ElementList querySelectorAll(String selectors) => document.querySelectorAll(selectors);
+
+/// 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);
+}