pkg/unittest: Run each test in a Zone

tests/lib/async/future_timeout_test.dart

Index: tests/lib/async/future_timeout_test.dart
diff --git a/tests/lib/async/future_timeout_test.dart b/tests/lib/async/future_timeout_test.dart
index cd306450f96d6c942ddc032ba161b262cbeffbb0..d81c87faca48a8686a0457e297932f65057eb6b4 100644
--- a/tests/lib/async/future_timeout_test.dart
+++ b/tests/lib/async/future_timeout_test.dart
@@ -142,6 +142,7 @@ main() {
   });
 
   test("timeoutZone", () {
+    var initialZone = Zone.current;
     Zone forked;
     int registerCallDelta = 0;
     bool callbackCalled = false;
@@ -169,7 +170,7 @@ main() {
     timedOut.then(expectAsync1((v) {
       expect(callbackCalled, true);
       expect(registerCallDelta, 0);
-      expect(Zone.current, Zone.ROOT);
+      expect(Zone.current, initialZone);
       expect(v, 42);
     }));
   });