Add the ability to abort a scheduled test.

pkg/scheduled_test/test/scheduled_test_test.dart

Index: pkg/scheduled_test/test/scheduled_test_test.dart
diff --git a/pkg/scheduled_test/test/scheduled_test_test.dart b/pkg/scheduled_test/test/scheduled_test_test.dart
index e849feadb8478d7ff9929e77cf09bfcc72552640..e42f810f90db9a15275e012593c372f3a541cbea 100644
--- a/pkg/scheduled_test/test/scheduled_test_test.dart
+++ b/pkg/scheduled_test/test/scheduled_test_test.dart
@@ -1359,4 +1359,83 @@ void main() {
       expect(errors.map((e) => e.error), equals(['error']));
     });
   }, passing: ['test 2']);
+
+  expectTestsPass("aborting the schedule before it's started running should "
+      "cause no tasks to be run", () {
+    test('test', () {
+      schedule(() {
+        throw 'error';
+      });
+
+      currentSchedule.abort();
+    });
+  });
+
+  expectTestsPass("aborting the schedule while it's running should stop future "
+      "tasks from running", () {
+    test('test', () {
+      schedule(currentSchedule.abort);
+
+      schedule(() {
+        throw 'error';
+      });
+    });
+  });
+
+  expectTestsPass("aborting the schedule while it's running shouldn't stop "
+      "tasks in other queues from running", () {
+    var onCompleteRun = false;
+    test('test 1', () {
+      schedule(currentSchedule.abort);
+
+      currentSchedule.onComplete.schedule(() {
+        onCompleteRun = true;
+      });
+    });
+
+    test('test 2', () {
+      expect(onCompleteRun, isTrue);
+    });
+  });
+
+  expectTestsPass("aborting the schedule while it's running shouldn't stop "
+      "out-of-band callbacks", () {
+    test('test', () {
+      var outOfBandFinished = false;
+      schedule(() {
+        wrapFuture(pumpEventQueue().then((_) {
+          outOfBandFinished = true;
+        }));
+
+        currentSchedule.abort();
+      });
+
+      currentSchedule.onComplete.schedule(() {
+        expect(outOfBandFinished, isTrue);
+      });
+    });
+  });
+
+  expectTestsPass("aborting the schedule in a non-tasks queue should stop "
+      "future tasks from running", () {
+    test('test', () {
+      currentSchedule.onComplete.schedule(() {
+        currentSchedule.abort();
+      });
+
+      currentSchedule.onComplete.schedule(() {
+        throw 'error';
+      });
+    });
+  });
+
+  expectTestsFail("aborting the schedule after an out-of-band error should "
+      "still surface the error", () {
+    test('test', () {
+      schedule(() {
+        currentSchedule.signalError('error');
+        currentSchedule.abort();
+      });
+    });
+  });
 }