Export DOMException for `instanceof` checks

fetch.js

@@ -424,7 +424,7 @@ Response.redirect = function(url, status) {
   return new Response(null, {status: status, headers: {location: url}})
 }
 
-var DOMException = self.DOMException
+export var DOMException = self.DOMException
 try {
   new DOMException()
 } catch (err) {

test/test.js

@@ -1060,6 +1060,7 @@ exercise.forEach(function(exerciseMode) {
               assert.ok(false)
             },
             function(error) {
+              assert.instanceOf(error, WHATWGFetch.DOMException)
               assert.equal(error.name, 'AbortError')
             }
           )