Only install @@toStringTag on the prototype

This fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=28244, aligning Web IDL objects with the built-in ECMAScript objects (see #226), and updating the spec to match 1/4 browsers (Chrome) instead of 0/4 as it currently does.

index.bs

@@ -6408,16 +6408,10 @@ Unless otherwise specified, the \[[Prototype]] internal property
 of objects defined in this section is the <emu-val>Object</emu-val> prototype object.
 
 Some objects described in this section are defined to have a <dfn id="dfn-class-string" export>class string</dfn>,
-which is the string to include in the string returned from Object.prototype.toString.
+which is the string to include in the string returned from <code>Object.prototype.toString</code>.
 If an object has a class string, then the object must,
-at the time it is created, have a property whose name is the [=@@toStringTag=] symbol
-and whose value is the specified string.
-
-<p class="issue">
-    Should define whether [=@@toStringTag=] is writable, enumerable and configurable.
-    All [=@@toStringTag=] properties in the ES6 spec are non-writable and non-enumerable,
-    and configurable.
-</p>
+at the time it is created, have a property whose name is the [=@@toStringTag=] symbol,
+whose value is the specified string, and which is non-writable, non-enumerable, and configurable.
 
 If an object is defined to be a <dfn id="dfn-function-object" export>function object</dfn>, then
 it has characteristics as follows:
@@ -10269,8 +10263,7 @@ of [=inherited interfaces=] for any
 other interface that is declared with one of these attributes, then the [=interface prototype object=]
 must be an [=immutable prototype exotic object=].
 
-The [=class string=] of an [=interface prototype object=] is the concatenation of
-the [=interface=]’s [=identifier=] and the string “Prototype”.
+The [=class string=] of an [=interface prototype object=] is the [=interface=]’s [=identifier=].
 
 
 <h4 id="legacy-callback-interface-object">Legacy callback interface object</h4>
@@ -11195,10 +11188,6 @@ use standard ECMAScript Array iterator objects.
 When a [=default iterator object=] is first created,
 its index is set to 0.
 
-The [=class string=] of a [=default iterator object=] for a given [=interface=]
-is the result of concatenting the [=identifier=] of the [=interface=]
-and the string “Iterator”.
-
 
 <h5 id="es-iterator-prototype-object">Iterator prototype object</h5>
 
@@ -11260,7 +11249,7 @@ must be [=%IteratorPrototype%=].
 </div>
 
 The [=class string=] of an [=iterator prototype object=] for a given [=interface=]
-is the result of concatenting the [=identifier=] of the [=interface=]
+is the result of concatenating the [=identifier=] of the [=interface=]
 and the string “Iterator”.
 
 
@@ -11755,12 +11744,6 @@ updated to be the [=interface prototype object=]
 of the [=primary interface=]
 from the [=platform object=]’s newly associated global environment.
 
-The [=class string=] of
-a platform object that implements one or more interfaces
-must be the [=identifier=] of
-the [=primary interface=]
-of the platform object.
-
 
 <h4 id="platform-object-setprototypeof" oldids="platformobjectsetprototypeof">\[[SetPrototypeOf]]</h4>
 
@@ -12508,7 +12491,7 @@ have an internal \[[Prototype]] property whose value is [=%ErrorPrototype%=].
 
 The [=class string=] of the
 [=DOMException prototype object=]
-is “DOMExceptionPrototype”.
+is “DOMException”.
 
 There must be a property named “constructor”
 on the DOMException prototype object with attributes
@@ -12543,10 +12526,6 @@ property of a {{DOMException}}
 object must be the [=DOMException prototype object=]
 from the global environment the exception object is associated with.
 
-The [=class string=]
-of a {{DOMException}} object
-must be “DOMException”.
-
 Note: The intention is for DOMException objects to be just like the other
 various native <emu-val>Error</emu-val> objects that the
 ECMAScript specification defines, apart from responding differently