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

Author: JiaLiPassion

Diff for: lib/zone.ts

@@ -1267,7 +1267,13 @@ const Zone: ZoneType = (function(global: any) {
         }
       }
       if (nativeMicroTaskQueuePromise) {
-        nativeMicroTaskQueuePromise[symbolThen](drainMicroTaskQueue);
+        let nativeThen = 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);
       }