Migrated test block 168 to Dart 2.0.

tests/lib_2/async/catch_errors.dart

 library catch_errors;
 
 import 'dart:async';
 
-Stream catchErrors(void body()) {
+Stream catchErrors(dynamic body()) {
   StreamController controller;
 
   bool onError(e, st) {
     controller.add(e);
     return true;
   }
 
   void onListen() {
     runZoned(body, onError: onError);
   }
@@ skipping 30 matching lines @@
   }
   ZoneSpecification specification = new ZoneSpecification(
       handleUncaughtError: errorHandler, scheduleMicrotask: asyncHandler);
   Zone zone = Zone.current.fork(specification: specification);
   if (onError != null) {
     return zone.runGuarded(body);
   } else {
     return zone.run(body);
   }
 }