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

sdk/lib/async/zone.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.async;
 
 typedef dynamic ZoneCallback();
 typedef dynamic ZoneUnaryCallback(arg);
 typedef dynamic ZoneBinaryCallback(arg1, arg2);
 
@@ skipping 367 matching lines @@
    */
   void print(String line);
 
   /**
    * The error zone is the one that is responsible for dealing with uncaught
    * errors. Errors are not allowed to cross zones with different error-zones.
    */
   Zone get _errorZone;
 
   /**
+   * Call to enter the Zone.
+   *
+   * The previous current zone is returned.
+   */
+  static Zone _enter(Zone zone) {
+    assert(zone != null);
+    assert(!identical(zone, _current));
+    Zone previous = _current;
+    _current = zone;
+    return previous;
+  }
+
+  /**
+   * Call to leave the Zone.
+   *
+   * The previous Zone must be provided as `previous`.
+   */
+  static void _leave(Zone previous) {
+    assert(previous != null);
+    Zone._current = previous;
+  }
+
+  /**
    * Retrieves the zone-value associated with [key].
    *
    * If this zone does not contain the value looks up the same key in the
    * parent zone. If the [key] is not found returns `null`.
    */
   operator[](Symbol key);
 }
 
 class _ZoneDelegate implements ZoneDelegate {
   final _BaseZone _degelationTarget;
@@ skipping 277 matching lines @@
         print("Stack Trace: \n$trace\n");
       }
       throw error;
     });
   });
 }
 
 dynamic _rootRun(Zone self, ZoneDelegate parent, Zone zone, f()) {
   if (Zone._current == zone) return f();
 
-  Zone old = Zone._current;
+  Zone old = Zone._enter(zone);
   try {
-    Zone._current = zone;
     return f();
   } finally {
-    Zone._current = old;
+    Zone._leave(old);
   }
 }
 
 dynamic _rootRunUnary(Zone self, ZoneDelegate parent, Zone zone, f(arg), arg) {
   if (Zone._current == zone) return f(arg);
 
-  Zone old = Zone._current;
+  Zone old = Zone._enter(zone);
   try {
-    Zone._current = zone;
     return f(arg);
   } finally {
-    Zone._current = old;
+    Zone._leave(old);
   }
 }
 
 dynamic _rootRunBinary(Zone self, ZoneDelegate parent, Zone zone,
                        f(arg1, arg2), arg1, arg2) {
   if (Zone._current == zone) return f(arg1, arg2);
 
-  Zone old = Zone._current;
+  Zone old = Zone._enter(zone);
   try {
-    Zone._current = zone;
     return f(arg1, arg2);
   } finally {
-    Zone._current = old;
+    Zone._leave(old);
   }
 }
 
 ZoneCallback _rootRegisterCallback(
     Zone self, ZoneDelegate parent, Zone zone, f()) {
   return f;
 }
 
 ZoneUnaryCallback _rootRegisterUnaryCallback(
     Zone self, ZoneDelegate parent, Zone zone, f(arg)) {
@@ skipping 191 matching lines @@
                                    handleUncaughtError: errorHandler);
   }
   Zone zone = Zone.current.fork(specification: zoneSpecification,
                                 zoneValues: zoneValues);
   if (onError != null) {
     return zone.runGuarded(body);
   } else {
     return zone.run(body);
   }
 }