fix #569, request will cause updateTaskCount failed if we call abort multipletimes (#570)

the fix is check whether the xhr has been aborted before abort the request

Author: JiaLiPassion

Diff for: lib/browser/browser.ts

@@ -125,7 +125,10 @@ function patchXHR(window: any) {
         const task: Task = findPendingTask(self);
         if (task && typeof task.type == 'string') {
           // If the XHR has already completed, do nothing.
-          if (task.cancelFn == null) {
+          // If the XHR has already been aborted, do nothing.
+          // Fix #569, call abort multiple times before done will cause
+          // macroTask task count be negative number
+          if (task.cancelFn == null || (task.data && (<XHROptions>task.data).aborted)) {
             return;
           }
           task.zone.cancelTask(task);

Diff for: test/browser/XMLHttpRequest.spec.ts

@@ -195,4 +195,18 @@ describe('XMLHttpRequest', function() {
          };
        });
      });
+
+  it('should keep taskcount correctly when abort was called multiple times before request is done',
+     function() {
+       testZone.run(function() {
+         const req = new XMLHttpRequest();
+         req.open('get', '/', true);
+         req.send();
+         req.addEventListener('readystatechange', function(ev) {
+           if (req.readyState >= 2) {
+             req.abort();
+           }
+         });
+       });
+     });
 });