IDL dictionary: Throw a TypeError when an interface member conversion fails

Source/bindings/templates/dictionary_v8.cpp

Index: Source/bindings/templates/dictionary_v8.cpp
diff --git a/Source/bindings/templates/dictionary_v8.cpp b/Source/bindings/templates/dictionary_v8.cpp
index 652ebc83bd4cd550581c38dd69b1ac1da9d7124f..c2e6ef1b96f7e498460f080a608721fb1d1aa5be 100644
--- a/Source/bindings/templates/dictionary_v8.cpp
+++ b/Source/bindings/templates/dictionary_v8.cpp
@@ -52,6 +52,12 @@ void {{v8_class}}::toImpl(v8::Isolate* isolate, v8::Handle<v8::Value> v8Value, {
         {{member.cpp_type}} {{member.name}};
         {% endif %}
         {{member.v8_value_to_local_cpp_value}};
+        {% if member.is_interface_type %}
+        if (!{{member.name}} && !{{member.name}}Value->IsNull()) {
+            exceptionState.throwTypeError("member {{member.name}} is not of type {{member.idl_type}}.");
+            return;
+        }
+        {% endif %}
         {% if member.enum_validation_expression %}
         String string = {{member.name}};
         if (!({{member.enum_validation_expression}})) {