Add isActive field on Timer.

sdk/lib/_internal/lib/isolate_helper.dart

 // Copyright (c) 2012, 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.
 
 library _isolate_helper;
 
 import 'dart:async';
 import 'dart:collection' show Queue, HashMap;
 import 'dart:isolate';
 import 'dart:_js_helper' show convertDartClosureToJS,
@@ skipping 1342 matching lines @@
 }
 
 class TimerImpl implements Timer {
   final bool _once;
   bool _inEventLoop = false;
   int _handle;
 
   TimerImpl(int milliseconds, void callback())
       : _once = true {
     if (milliseconds == 0 && (!hasTimer() || _globalState.isWorker)) {
+
+      void internalCallback() {
+        _handle = null;
+        callback();
+      }
+
+      // Setting _handle to something different from null indicates that the
+      // callback has not been run. Hence, the choice of 1 is arbitrary.
+      _handle = 1;
+
       // This makes a dependency between the async library and the
       // event loop of the isolate library. The compiler makes sure
       // that the event loop is compiled if [Timer] is used.
       // TODO(7907): In case of web workers, we need to use the event
       // loop instead of setTimeout, to make sure the futures get executed in
       // order.
       _globalState.topEventLoop.enqueue(
-          _globalState.currentContext, callback, 'timer');
+          _globalState.currentContext, internalCallback, 'timer');
       _inEventLoop = true;
     } else if (hasTimer()) {
-      _globalState.topEventLoop.activeTimerCount++;
+
       void internalCallback() {
-        callback();
         _handle = null;
         _globalState.topEventLoop.activeTimerCount--;
+        callback();
       }
+
+      _globalState.topEventLoop.activeTimerCount++;
       _handle = JS('int', '#.setTimeout(#, #)',
                    globalThis,
                    convertDartClosureToJS(internalCallback, 0),
                    milliseconds);
     } else {
       assert(milliseconds > 0);
       throw new UnsupportedError("Timer greater than 0.");
     }
   }
 
@@ skipping 20 matching lines @@
       if (_once) {
         JS('void', '#.clearTimeout(#)', globalThis, _handle);
       } else {
         JS('void', '#.clearInterval(#)', globalThis, _handle);
       }
       _handle = null;
     } else {
       throw new UnsupportedError("Canceling a timer.");
     }
   }
+
+  bool get isActive => _handle != null;
 }
 
 bool hasTimer() => JS('', '#.setTimeout', globalThis) != null;