Treat non-callable input as null for EventHandler attributes

Source/bindings/v8/V8EventListener.cpp

Index: Source/bindings/v8/V8EventListener.cpp
diff --git a/Source/bindings/v8/V8EventListener.cpp b/Source/bindings/v8/V8EventListener.cpp
index 8ada755b27a3e233f0a5002bafcd30e3ec8f75f2..343654c98584d1d07599b829dabed6ab236f8972 100644
--- a/Source/bindings/v8/V8EventListener.cpp
+++ b/Source/bindings/v8/V8EventListener.cpp
@@ -55,6 +55,7 @@ v8::Local<v8::Function> V8EventListener::getListenerFunction(ScriptExecutionCont
         return v8::Local<v8::Function>::Cast(listener);
 
     if (listener->IsObject()) {
+        ASSERT_WITH_MESSAGE(!isAttribute(), "EventHandler attributes should only accept JS Functions as input.");
         v8::Local<v8::Value> property = listener->Get(v8::String::NewSymbol("handleEvent"));
         // Check that no exceptions were thrown when getting the
         // handleEvent property and that the value is a function.