Fixing overrideMimeType on IE10.

tools/dom/templates/html/impl/impl_XMLHttpRequest.darttemplate

Index: tools/dom/templates/html/impl/impl_XMLHttpRequest.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_XMLHttpRequest.darttemplate b/tools/dom/templates/html/impl/impl_XMLHttpRequest.darttemplate
index e130052c8747f4c7cee0b821e0c926f196ec6bd8..b1fee6e85f0b5d49ed198eeac87e7b8f02d8f5b1 100644
--- a/tools/dom/templates/html/impl/impl_XMLHttpRequest.darttemplate
+++ b/tools/dom/templates/html/impl/impl_XMLHttpRequest.darttemplate
@@ -174,5 +174,18 @@ $else
 $endif
   }
 
+  /**
+   * Checks to see if the overrideMimeType method is supported on the current
+   * platform.
+   */
+  static bool get supportsOverrideMimeType {
+$if DART2JS
+    var xhr = new HttpRequest();
+    return JS('bool', '("overrideMimeType" in #)', xhr);
+$else
+    return true;
+$endif
+  }
+
 $!MEMBERS
 }