Use same condition in Body and Request for choosing responseType.

Chrome will error on access to xhr.responseText when xhr.responseType is set to something other than 'text'. Everything that supports Blob should support XHR2.

fetch.js

@@ -69,20 +69,20 @@
     body.bodyUsed = true
   }
 
+  var blobSupport = self.FileReader && (function() {
+    try {
+      new Blob();
+      return true
+    } catch(e) {
+      return false
+    }
+  })();
+
   function Body() {
     this._body = null
     this.bodyUsed = false
 
-    var blobSupport = (function() {
-      try {
-        new Blob();
-        return true
-      } catch(e) {
-        return false
-      }
-    })();
-
-    if (blobSupport && self.FileReader) {
+    if (blobSupport) {
       this.arrayBuffer = function() {
         var rejected = consumed(this)
         if (rejected) {
@@ -209,7 +209,7 @@
           headers: headers(xhr),
           url: xhr.responseURL || xhr.getResponseHeader('X-Request-URL')
         }
-        resolve(new Response('responseType' in xhr ? xhr.response: xhr.responseText, options))
+        resolve(new Response(blobSupport ? xhr.response: xhr.responseText, options))
       }
 
       xhr.onerror = function() {
@@ -217,7 +217,7 @@
       }
 
       xhr.open(self.method, self.url)
-      if ('responseType' in xhr) {
+      if (blobSupport) {
         xhr.responseType = 'blob'
       }