Introduce and use Zone:_enter and Zone:_leave, in Future.

tests/lib/async/zone_run_test.dart

Index: tests/lib/async/zone_run_test.dart
diff --git a/tests/lib/async/zone_run_test.dart b/tests/lib/async/zone_run_test.dart
index 3c13c469f082de94000e2b5b1fc4e82eb922232a..28bd2b798c6a350015b38d96a18a2f61051e90c1 100644
--- a/tests/lib/async/zone_run_test.dart
+++ b/tests/lib/async/zone_run_test.dart
@@ -62,4 +62,25 @@ main() {
         events);
     asyncEnd();
   });
+
+  var zone1 = Zone.ROOT.fork();
+  var zone2 = Zone.ROOT.fork();
+  var zone3 = Zone.ROOT.fork();
+  asyncStart();
+  asyncStart();
+  zone1.run(() {
+    var future = new Future.value();
+    zone2.run(() {
+      future.then((_) {
+        Expect.identical(zone2, Zone.current);
+        asyncEnd();
+      });
+    });
+    zone3.run(() {
+      future.then((_) {
+        Expect.identical(zone3, Zone.current);
+        asyncEnd();
+      });
+    });
+  });
 }