Make sure handler errors won't bring down a shelf server.

pkg/shelf/lib/src/util.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library shelf.util;
 
 import 'dart:async';
 
 import 'package:stack_trace/stack_trace.dart';
 
 /// Like [Future.sync], but wraps the Future in [Chain.track] as well.
 Future syncFuture(callback()) => Chain.track(new Future.sync(callback));
+
+/// Run [callback] and capture any errors that would otherwise be top-leveled.
+///
+/// If [this] is called in a non-root error zone, it will just run [callback]
+/// and return the result. Otherwise, it will capture any errors using
+/// [runZoned] and pass them to [onError].
+catchTopLevelErrors(callback(), void onError(error, StackTrace stackTrace)) {
+  if (Zone.current.inSameErrorZone(Zone.ROOT)) {
+    return runZoned(callback, onError: onError);
+  } else {
+    return callback();
+  }
+}