Revert "Zone support for Futures."

tests/lib/async/catch_errors.dart

-library catch_errors;
-
-import 'dart:async';
-
-Stream catchErrors(void body()) {
-  StreamController controller;
-
-  bool onError(e) {
-    controller.add(e);
-    return true;
-  }
-
-  void onDone() {
-    controller.close();
-  }
-
-  void onListen() {
-    runZonedExperimental(body, onError: onError, onDone: onDone);
-  }
-
-  controller = new StreamController(onListen: onListen);
-  return controller.stream;
-}
-
-Future waitForCompletion(void body()) {
-  Completer completer = new Completer.sync();
-  runZonedExperimental(body, onDone: completer.complete);
-  return completer.future;
-}