Fixing overrideMimeType on IE10.

tests/html/xhr_test.dart

Index: tests/html/xhr_test.dart
diff --git a/tests/html/xhr_test.dart b/tests/html/xhr_test.dart
index a3d7a852155ce289befaf9ad65d79ce16fcaa05c..484b2e7642e7860313aea67d43c62c31b1f125c3 100644
--- a/tests/html/xhr_test.dart
+++ b/tests/html/xhr_test.dart
@@ -51,6 +51,12 @@ main() {
     });
   });
 
+  group('supported_overrideMimeType', () {
+    test('supported', () {
+      expect(HttpRequest.supportsOverrideMimeType, isTrue);
+    });
+  });
+
   group('xhr', () {
     test('XHR No file', () {
       HttpRequest xhr = new HttpRequest();
@@ -146,7 +152,6 @@ 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));
@@ -165,6 +170,15 @@ main() {
         expect(event is HttpRequestProgressEvent, isTrue);
       }, expectation);
     });
+
+    test('overrideMimeType', () {
+      var expectation =
+          HttpRequest.supportsOverrideMimeType ? returnsNormally : throws;
+
+      expect(() {
+        HttpRequest.request(url, mimeType: 'application/binary');
+      }, expectation);
+    });
   });
 
   group('xhr_requestBlob', () {