UserAgent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Ubuntu Chromium/56.0.2924.76 Chrome/56.0.2924.76 Safari/537.36

Steps to reproduce the problem:
1. Execute the following code in the context of a service worker:

       new ExtendableMessageEvent('name', { ports: null });

What is the expected behavior?
A TypeError should be thrown

What went wrong?
The expression results with a normal completion, the value of which is an instance of ExtendableMessageEvent whose `ports` property is `null`.

Did this work before? N/A 

Does this work in other browsers? Yes

Chrome version: 58.0.3029.19  Channel: dev
OS Version: 
Flash Version: 

According to the WebIDL definition [1], the value is not nullable and must be
coercible to a sequence.

The project currently includes an invalid test that asserts the incorrect
behavior as "expected" [2]. I am currently working to migrate these tests to
the Web Platform Tests project (as per [3]); I am correcting this test and
blacklisting it in the process.

[1] https://w3c.github.io/ServiceWorker/#extendablemessageevent-interface

> # 4.8. ExtendableMessageEvent
>
>     dictionary ExtendableMessageEventInit : ExtendableEventInit {
>       any data = null;
>       USVString origin = "";
>       DOMString lastEventId = "";
>       (Client or ServiceWorker or MessagePort)? source = null;
>       sequence<MessagePort> ports = [];
>     };

[2] https://cs.chromium.org/chromium/src/third_party/WebKit/LayoutTests/http/tests/serviceworker/ServiceWorkerGlobalScope/resources/extendable-message-event-constructor-worker.js?sq&l=134
[3] https://bugs.chromium.org/p/chromium/issues/detail?id=688116&desc=2