Fixing overrideMimeType on IE10.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 94ee92e5598f0d56f02cf9cd3499cb18447b091b..0b423020c3d9f642976bfd63ce9711e4a96bc716 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -10922,6 +10922,15 @@ class HttpRequest extends EventTarget native "XMLHttpRequest" {
     return JS('bool', '("onloadend" in #)', xhr);
   }
 
+  /**
+   * Checks to see if the overrideMimeType method is supported on the current
+   * platform.
+   */
+  static bool get supportsOverrideMimeType {
+    var xhr = new HttpRequest();
+    return JS('bool', '("overrideMimeType" in #)', xhr);
+  }
+
 
   @DomName('XMLHttpRequest.abortEvent')
   @DocsEditable
@@ -11178,6 +11187,9 @@ class HttpRequest extends EventTarget native "XMLHttpRequest" {
    */
   @DomName('XMLHttpRequest.overrideMimeType')
   @DocsEditable
+  @SupportedBrowser(SupportedBrowser.CHROME)
+  @SupportedBrowser(SupportedBrowser.FIREFOX)
+  @SupportedBrowser(SupportedBrowser.SAFARI)
   void overrideMimeType(String override) native;
 
   @JSName('removeEventListener')