Align with IDL constructor changes

Fixes #929.

fetch.bs

@@ -5152,16 +5152,18 @@ fetch("/music/pk/altes-kamuffel.flac")
 <pre class=idl>
 typedef (sequence&lt;sequence&lt;ByteString>> or record&lt;ByteString, ByteString>) HeadersInit;
 
-[Constructor(optional HeadersInit init),
- Exposed=(Window,Worker)]
+[Exposed=(Window,Worker)]
 interface Headers {
+  constructor(optional HeadersInit init);
+
   void append(ByteString name, ByteString value);
   void delete(ByteString name);
   ByteString? get(ByteString name);
   boolean has(ByteString name);
   void set(ByteString name, ByteString value);
   iterable&lt;ByteString, ByteString>;
-};</pre>
+};
+</pre>
 
 <p class=note>Unlike a <a for=/>header list</a>, a {{Headers}} object cannot represent more than one
 `<code>Set-Cookie</code>` <a for=/>header</a>. In a way this is problematic as unlike all other
@@ -5689,9 +5691,10 @@ invoked, must return the result of running
 <pre class=idl>
 typedef (Request or USVString) RequestInfo;
 
-[Constructor(RequestInfo input, optional RequestInit init = {}),
- Exposed=(Window,Worker)]
+[Exposed=(Window,Worker)]
 interface Request {
+  constructor(RequestInfo input, optional RequestInit init = {});
+
   readonly attribute ByteString method;
   readonly attribute USVString url;
   [SameObject] readonly attribute Headers headers;
@@ -5733,7 +5736,8 @@ enum RequestDestination { "", "audio", "audioworklet", "document", "embed", "fon
 enum RequestMode { "navigate", "same-origin", "no-cors", "cors" };
 enum RequestCredentials { "omit", "same-origin", "include" };
 enum RequestCache { "default", "no-store", "reload", "no-cache", "force-cache", "only-if-cached" };
-enum RequestRedirect { "follow", "error", "manual" };</pre>
+enum RequestRedirect { "follow", "error", "manual" };
+</pre>
 
 <p class="note no-backref">"<code>serviceworker</code>" is omitted from
 <a enum><code>RequestDestination</code></a> as it cannot be observed from JavaScript. Implementations
@@ -6336,9 +6340,10 @@ run these steps:
 
 <h3 id=response-class>Response class</h3>
 
-<pre class=idl>[Constructor(optional BodyInit? body = null, optional ResponseInit init = {}),
- Exposed=(Window,Worker)]
+<pre class=idl>[Exposed=(Window,Worker)]
 interface Response {
+  constructor(optional BodyInit? body = null, optional ResponseInit init = {});
+
   [NewObject] static Response error();
   [NewObject] static Response redirect(USVString url, optional unsigned short status = 302);
 
@@ -6362,7 +6367,8 @@ dictionary ResponseInit {
   HeadersInit headers;
 };
 
-enum ResponseType { "basic", "cors", "default", "error", "opaque", "opaqueredirect" };</pre>
+enum ResponseType { "basic", "cors", "default", "error", "opaque", "opaqueredirect" };
+</pre>
 
 <p>A {{Response}} object has an associated
 <dfn id=concept-response-response for=Response export>response</dfn> (a
@@ -6559,8 +6565,9 @@ run these steps:
 
 <pre class=idl>
 partial interface mixin WindowOrWorkerGlobalScope {
-  [NewObject] Promise&lt;Response> fetch(RequestInfo input, optional RequestInit init);
-};</pre>
+  [NewObject] Promise&lt;Response> fetch(RequestInfo input, optional RequestInit init = {});
+};
+</pre>
 
 <p>The
 <dfn id=dom-global-fetch method for=WindowOrWorkerGlobalScope><code>fetch(<var>input</var>, <var>init</var>)</code></dfn>