Modify custom elements to inherit from HtmlElement

tools/dom/src/native_DOMImplementation.dart

Index: tools/dom/src/native_DOMImplementation.dart
diff --git a/tools/dom/src/native_DOMImplementation.dart b/tools/dom/src/native_DOMImplementation.dart
index 8cdf24d8e3edc1a85b4d60aad7253bdcede07677..5b4f1e4cfa650cf2d6c16ddd6fff6db33e2ed8f0 100644
--- a/tools/dom/src/native_DOMImplementation.dart
+++ b/tools/dom/src/native_DOMImplementation.dart
@@ -211,7 +211,8 @@ class _Utils {
     Symbol objectName = reflectClass(Object).qualifiedName;
     bool isRoot(ClassMirror cls) =>
       cls == null || cls.qualifiedName == objectName;
-    Symbol elementName = reflectClass(Element).qualifiedName;
+    // TODO(vsm): Support extending SvgElement as well.
+    Symbol elementName = reflectClass(HtmlElement).qualifiedName;
     bool isElement(ClassMirror cls) =>
       cls != null && cls.qualifiedName == elementName;
 
@@ -220,7 +221,7 @@ class _Utils {
     }
 
     if (isRoot(superClass)) {
-      throw new UnsupportedError("Invalid custom element doesn't inherit from Element.");
+      throw new UnsupportedError("Invalid custom element doesn't inherit from HtmlElement.");
     }
     _register(tag, type);
   }