Extension support

tools/dom/templates/html/dartium/html_dartium.darttemplate

Index: tools/dom/templates/html/dartium/html_dartium.darttemplate
diff --git a/tools/dom/templates/html/dartium/html_dartium.darttemplate b/tools/dom/templates/html/dartium/html_dartium.darttemplate
index 9bd24f0b78f54de6e6ea1ec344a701fb2b2698b2..2d157eab2139761fc4c650918f88da1636251fd0 100644
--- a/tools/dom/templates/html/dartium/html_dartium.darttemplate
+++ b/tools/dom/templates/html/dartium/html_dartium.darttemplate
@@ -353,13 +353,30 @@ Function _getSvgFunction(String key) {
  ******************************************************************************/
 
 // List of known tagName to DartClass for custom elements, used for upgrade.
-var _knownCustomElements = new Map<String, Type>();
+var _knownCustomElements = new Map<String, Map<Type, String>>();
 
-void _addCustomElementType(String tagName, Type dartClass) {
-  _knownCustomElements[tagName] = dartClass;
+void _addCustomElementType(String tagName, Type dartClass, [String extendTag]) {
+  _knownCustomElements[tagName] = 
+      {'type': dartClass, 'extends': extendTag != null ? extendTag : "" };
 }
 
-Type _getCustomElementType(object) => _knownCustomElements[_getCustomElementName(object)];
+Type _getCustomElementType(object) {
+  var entry = _knownCustomElements[_getCustomElementName(object)];
+  if (entry != null) {
+    return entry['type'];
+  }
+  return null;
+}
+
+String _getCustomElementExtends(object) {
+  var entry = _knownCustomElements[_getCustomElementName(object)];
+  if (entry != null) {
+    return entry['extends'];
+  }
+  return null;
+}
+
+_getCustomElement(object) => _knownCustomElements[_getCustomElementName(object)];
 
 // Return the tag name or is attribute of the custom element or data binding.
 String _getCustomElementName(element) {
@@ -488,9 +505,15 @@ wrap_jso(jsObject) {
       // make sure it's upgraded.
       dartClass_instance = _upgradeHtmlElement(jsObject['dart_class']);
     } else {
-      var customElementClass = _getCustomElementType(jsObject);
+      var customElementClass = null;
+      var extendsTag = "";
+      var custom = _getCustomElement(jsObject);
+      if (custom != null) {
+        customElementClass = custom['type'];
+        extendsTag = custom['extends'];
+      }
       // Custom Element to upgrade.
-      if (jsTypeName == 'HTMLElement' && customElementClass != null) {
+      if (jsTypeName == 'HTMLElement' && customElementClass != null && extendsTag == "") {
         try {
           dartClass_instance = _blink.Blink_Utils.constructElement(customElementClass, jsObject);
         } finally {