Add [[ErrorData]] slot to DOMExceptions

Fixes tc39/proposal-is-error#9.

index.bs

@@ -13538,8 +13538,9 @@ the realm given as an argument.
             1.  Let |targetRealm| be [=?=] [$GetFunctionRealm$](|newTarget|).
             1.  Set |prototype| to the [=interface prototype object=] for |interface| in
                 |targetRealm|.
-    1.  Let |instance| be [$MakeBasicObject$](
-        « \[[Prototype]], \[[Extensible]], \[[Realm]], \[[PrimaryInterface]] »).
+    1.  Let |slots| be « \[[Prototype]], \[[Extensible]], \[[Realm]], \[[PrimaryInterface]] ».
+    1.  If |interface| is {{DOMException}}, append \[[ErrorData]] to |slots|.
+    1.  Let |instance| be [$MakeBasicObject$](|slots|).
     1.  Set |instance|.\[[Realm]] to |realm|.
     1.  Set |instance|.\[[PrimaryInterface]] to |interface|.
     1.  Set |instance|.\[[Prototype]] to |prototype|.
@@ -14553,6 +14554,7 @@ The [=class string=] of a [=namespace object=] is the [=namespace=]'s [=identifi
 In the JavaScript binding, the [=interface prototype object=] for {{DOMException}}
 has its \[[Prototype]] [=/internal slot=] set to the intrinsic object {{%Error.prototype%}},
 as defined in the [=create an interface prototype object=] abstract operation.
+It also has an \[[ErrorData]] slot, like all built-in exceptions.
 
 Additionally, if an implementation gives native {{Error}} objects special powers or
 nonstandard properties (such as a <code>stack</code> property),