Throw, instead of assert, for transferring detached ArrayBuffers

This makes the algorithm more user-friendly, as this is generally the desired behavior, and the algorithm can already throw for other reasons so callers are expected to handle that.

Author: domenic

index.bs

@@ -9075,7 +9075,8 @@ a reference to the same object that the IDL value represents.
 
     1.  Let |jsArrayBuffer| be the result of [=converted to a JavaScript value|converting=]
         |arrayBuffer| to a JavaScript value.
-    1.  Assert: [$IsDetachedBuffer$](|jsArrayBuffer|) is false.
+    1.  If [$IsDetachedBuffer$](|jsArrayBuffer|) is false, then [=JavaScript/throw=] a
+        <l spec=ecmascript>{{TypeError}}</l>.
     1.  Let |arrayBufferData| be |jsArrayBuffer|.\[[ArrayBufferData]].
     1.  Let |arrayBufferByteLength| be |jsArrayBuffer|.\[[ArrayBufferByteLength]].
     1.  Perform [=?=] [$DetachArrayBuffer$](|jsArrayBuffer|).
@@ -9088,7 +9089,8 @@ a reference to the same object that the IDL value represents.
         value of type {{ArrayBuffer}}.
 
     <p class="note">This will throw an exception under the same circumstances as
-    [=ArrayBuffer/detaching=].
+    [=ArrayBuffer/detaching=], and also for {{ArrayBuffer}}s that are already
+    [=BufferSource/detached=].
 </div>
 
 <h4 id="js-frozen-array" oldids="es-frozen-array">Frozen arrays — FrozenArray&lt;|T|&gt;</h4>