Throw a TypeError if Request or Response functions are called without…

… `new`

fetch.js

@@ -322,6 +322,10 @@ function normalizeMethod(method) {
 }
 
 export function Request(input, options) {
+  if (!(this instanceof Request)) {
+    throw new TypeError('Please use the "new" operator, this DOM object constructor cannot be called as a function.')
+  }
+
   options = options || {}
   var body = options.body
 
@@ -414,6 +418,9 @@ function parseHeaders(rawHeaders) {
 Body.call(Request.prototype)
 
 export function Response(bodyInit, options) {
+  if (!(this instanceof Response)) {
+    throw new TypeError('Please use the "new" operator, this DOM object constructor cannot be called as a function.')
+  }
   if (!options) {
     options = {}
   }

test/test.js

@@ -330,6 +330,11 @@ exercise.forEach(function(exerciseMode) {
 
     // https://fetch.spec.whatwg.org/#request-class
     suite('Request', function() {
+      test('called as normal function', function() {
+        assert.throws(function() {
+          Request('https://fetch.spec.whatwg.org/')
+        })
+      })
       test('construct with string url', function() {
         var request = new Request('https://fetch.spec.whatwg.org/')
         assert.equal(request.url, 'https://fetch.spec.whatwg.org/')
@@ -612,6 +617,11 @@ exercise.forEach(function(exerciseMode) {
         return new Response(body)
       })
 
+      test('called as normal function', function() {
+        assert.throws(function() {
+          Response('{"foo":"bar"}', {headers: {'content-type': 'application/json'}})
+        })
+      })
       test('creates Headers object from raw headers', function() {
         var r = new Response('{"foo":"bar"}', {headers: {'content-type': 'application/json'}})
         assert.equal(r.headers instanceof Headers, true)