Rewrite Zone implementation to avoid recursively looking up stuff in the parent chain.

tests/lib/async/run_zoned9_test.dart

Index: tests/lib/async/run_zoned9_test.dart
diff --git a/tests/lib/async/run_zoned9_test.dart b/tests/lib/async/run_zoned9_test.dart
index 0373cac3c2d3fcd4e14e6b48e71b6133866a4719..e76268f9abe9936efa988ab3e5feb5b422c834d6 100644
--- a/tests/lib/async/run_zoned9_test.dart
+++ b/tests/lib/async/run_zoned9_test.dart
@@ -25,11 +25,11 @@ main() {
       // If we are waiting for an error, don't asyncEnd, but let it time out.
       if (false) /// 01: runtime error
         asyncEnd();
-      throw e;  /// 01: runtime error
+      throw e;   /// 01: continued
     });
   } catch (e) {
     // We should never see an error here.
-    if (false)  /// 01: continued
+    if (false)   /// 01: continued
       rethrow;
   }
 }