Revert the revert.

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 920c422826b3ecb610c1a1bad6b7f3dc0e65a8c6..53b3e084741eded8863e63e1d84465b0f71a25cb 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -13202,7 +13202,19 @@ class HttpRequest extends EventTarget native "*XMLHttpRequest" {
    */
   static bool get supportsProgressEvent {
     var xhr = new HttpRequest();
-    return JS('bool', '"onprogress" in #', xhr);
+    return JS('bool', '("onprogress" in #)', xhr);
+  }
+
+  /**
+   * Checks to see if the current platform supports making cross origin
+   * requests.
+   *
+   * Note that even if cross origin requests are supported, they still may fail
+   * if the destination server does not support CORS requests.
+   */
+  static bool get supportsCrossOrigin {
+    var xhr = new HttpRequest();
+    return JS('bool', '("withCredentials" in #)', xhr);
   }
 
   /**
@@ -13210,7 +13222,7 @@ class HttpRequest extends EventTarget native "*XMLHttpRequest" {
    */
   static bool get supportsLoadEndEvent {
     var xhr = new HttpRequest();
-    return JS('bool', '"onloadend" in #', xhr);
+    return JS('bool', '("onloadend" in #)', xhr);
   }