Enforcing that custom tag extensions have correct super classes.

Source/bindings/dart/DartCustomElementConstructorBuilder.cpp

Index: Source/bindings/dart/DartCustomElementConstructorBuilder.cpp
diff --git a/Source/bindings/dart/DartCustomElementConstructorBuilder.cpp b/Source/bindings/dart/DartCustomElementConstructorBuilder.cpp
index 5d7e4475ec0b8745c1e9ef244bd12e2701a58640..0cec8815076954f7ab5dcb1d7bf64f43f50e0b66 100644
--- a/Source/bindings/dart/DartCustomElementConstructorBuilder.cpp
+++ b/Source/bindings/dart/DartCustomElementConstructorBuilder.cpp
@@ -79,6 +79,11 @@ bool DartCustomElementConstructorBuilder::validateOptions(const AtomicString& ty
     if (!m_extendsTagName.isNull() && !m_extendsTagName.isEmpty()) {
         localName = m_extendsTagName.lower();
 
+        if (!DartUtilities::isTypeSubclassOfTag(m_customType, m_extendsTagName)) {
+            CustomElementException::throwException(CustomElementException::PrototypeDoesNotExtendHTMLElementSVGElementPrototype, type, es);
+            return false;
+        }
+
         // TODO: enable once we pick up Blink version 31
         // if (!Document::isValidName(localName)) {
         //     CustomElementException::throwException(CustomElementException::ExtendsIsInvalidName, type, es);