Add isActive field on Timer.

tests/lib/async/timer_isActive_test.dart

+// Copyright (c) 2011, 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.
+
+import 'dart:async';
+import '../../../pkg/unittest/lib/unittest.dart';
+
+main() {
+
+  test("timer isActive test", () {
+    Timer t;
+
+    t = new Timer(const Duration(seconds: 1),
+                  expectAsync0(() => expect(t.isActive, equals(false))));
+    expect(t.isActive, equals(true));
+  });
+
+  test("periodic timer cancel test", () {
+    Timer t;
+
+    int i = 0;
+    void checkActive(Timer timer) {
+      expect(t.isActive, equals(true));
+      if (i == 2) {
+        t.cancel();
+        expect(t.isActive, equals(false));
+      }
+      i++;
+    }
+
+    t = new Timer.periodic(new Duration(milliseconds: 1),
+                           expectAsync1(checkActive, count: 3));
+    expect(t.isActive, equals(true));
+  });
+
+  test("timer cancel test", () {
+    Timer timer = new Timer(const Duration(seconds: 1),
+                            () => fail("Should not be reached."));
+    Timer.run(expectAsync0(() {
+      expect(timer.isActive, equals(true));
+      timer.cancel();
+      expect(timer.isActive, equals(false));
+    }));
+  });
+}