fix bug add event listener to xhrhttprequest multiple times

lib/browser/browser.ts

@@ -48,6 +48,7 @@ patchXHR(_global);
 
 const XHR_TASK = zoneSymbol('xhrTask');
 const XHR_SYNC = zoneSymbol('xhrSync');
+const XHR_LISTENER = zoneSymbol('xhrListener');
 
 interface XHROptions extends TaskData {
   target: any;
@@ -63,13 +64,20 @@ function patchXHR(window: any) {
 
   function scheduleTask(task: Task) {
     var data = <XHROptions>task.data;
-    data.target.addEventListener('readystatechange', () => {
+    // remove existing event listener
+    var listener = data.target[XHR_LISTENER];
+    if (listener) {
+        data.target.removeEventListener('readystatechange', listener);
+    }
+    var newListener = data.target[XHR_LISTENER] = () => {
       if (data.target.readyState === data.target.DONE) {
         if (!data.aborted) {
           task.invoke();
         }
       }
-    });
+    };
+    data.target.addEventListener('readystatechange', newListener);
+
     var storedTask: Task = data.target[XHR_TASK];
     if (!storedTask) {
       data.target[XHR_TASK] = task;

test/browser/XMLHttpRequest.spec.ts

@@ -182,4 +182,16 @@ describe('XMLHttpRequest', function() {
     expect(XMLHttpRequest.LOADING).toEqual(3);
     expect(XMLHttpRequest.DONE).toEqual(4);
   });
+
+  it('should work properly when send request multiple times on single xmlRequest instance', function() {
+    testZone.run(function() {
+      var req = new XMLHttpRequest();
+      req.open('get', '/', true);
+      req.send();
+      req.onloadend = function() {
+        req.open('get', '/', true);
+        req.send();
+      }
+    });
+  })
 });