Moving overrideMimeType to depend on typed data.

Fixes IE9 test failure of overrideMimeType.
BUG=
R=efortuna@google.com

Review URL: https://codereview.chromium.org//14651023

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@22593 260f80e4-7a28-3924-810f-c04153c831b5

tests/html/xhr_test.dart

@@ -146,7 +146,6 @@ main() {
     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));
@@ -170,7 +169,8 @@ main() {
   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;