Moving overrideMimeType to depend on typed data.

tests/html/xhr_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library XHRTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_individual_config.dart';
 import 'dart:async';
 import 'dart:html';
 import 'dart:json' as json;
@@ skipping 128 matching lines @@
       HttpRequest.getString('NonExistingFile').then(
         (_) { fail('Succeeded for non-existing file.'); },
         onError: expectAsync1((error) {
           validate404(error.target);
         }));
     });
 
     test('XHR.request responseType arraybuffer', () {
       if (Platform.supportsTypedData) {
         HttpRequest.request(url, responseType: 'arraybuffer',
-          mimeType: 'application/binary',
           requestHeaders: {'Content-Type': 'text/xml'}).then(
           expectAsync1((xhr) {
             expect(xhr.status, equals(200));
             var byteBuffer = xhr.response;
             expect(byteBuffer, new isInstanceOf<ByteBuffer>());
             expect(byteBuffer, isNotNull);
           }));
       }
     });
 
     test('HttpRequestProgressEvent', () {
       var expectation = HttpRequestProgressEvent.supported ?
           returnsNormally : throws;
       expect(() {
         var event = new Event.eventType('XMLHttpRequestProgressEvent', '');
         expect(event is HttpRequestProgressEvent, isTrue);
       }, expectation);
     });
   });
 
   group('xhr_requestBlob', () {
     test('XHR.request responseType blob', () {
       if (Platform.supportsTypedData) {
-        return HttpRequest.request(url, responseType: 'blob').then(
+        return HttpRequest.request(url, responseType: 'blob',
+          mimeType: 'application/binary').then(
           (xhr) {
             expect(xhr.status, equals(200));
             var blob = xhr.response;
             expect(blob is Blob, isTrue);
             expect(blob, isNotNull);
           });
       }
     });
   });
 }