fix(core): use then directly when promise is not patchable (#1079)

Author: JiaLiPassion

lib/zone.ts

@@ -1267,7 +1267,13 @@ const Zone: ZoneType = (function(global: any) {
 }
 }
 if(nativeMicroTaskQueuePromise){
-nativeMicroTaskQueuePromise[symbolThen](drainMicroTaskQueue);
+letnativeThen=nativeMicroTaskQueuePromise[symbolThen];
+if(!nativeThen){
+// native Promise is not patchable, we need to use `then` directly
+// issue 1078
+nativeThen=nativeMicroTaskQueuePromise['then'];
+}
+nativeThen.call(nativeMicroTaskQueuePromise,drainMicroTaskQueue);
 }else{
 global[symbolSetTimeout](drainMicroTaskQueue,0);
 }