Add a shelf handler for running one-off handlers.

lib/src/util/one_off_handler.dart

Index: lib/src/util/one_off_handler.dart
diff --git a/lib/src/util/one_off_handler.dart b/lib/src/util/one_off_handler.dart
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index 0000000000000000000000000000000000000000..bffefe8dc63a9ad224315f27a412ee0a76c3beae
--- /dev/null
+++ b/lib/src/util/one_off_handler.dart
@@ -0,0 +1,47 @@
+// Copyright (c) 2015, 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 unittest.one_off_handler;
+
+import 'package:path/path.dart' as p;
+import 'package:shelf/shelf.dart' as shelf;
+
+/// A Shelf handler that provides support for one-time handlers.
+///
+/// This is useful for handlers that only expect to be hit once before becoming
+/// invalid and don't need to have a persistent URL.
+class OneOffHandler {
+  /// A map from URL paths to handlers.
+  final _handlers = new Map<String, shelf.Handler>();
+
+  /// The counter of handlers that have been activated.
+  var _counter = 0;
+
+  /// The actual [shelf.Handler] that dispatches requests.
+  shelf.Handler get handler => _onRequest;
+
+  /// Creates a new one-off handler that forwards to [handler].
+  ///
+  /// Returns a string that's the URL path for hitting this handler, relative to
+  /// the URL for the one-off handler itself.
+  ///
+  /// [handler] will be unmounted as soon as it receives a request.
+  String create(shelf.Handler handler) {
+    var path = _counter.toString();
+    _handlers[path] = handler;
+    _counter++;
+    return path;
+  }
+
+  /// Dispatches [request] to the appropriate handler.
+  _onRequest(shelf.Request request) {
+    // Skip the first component because it's always "/".
+    var components = p.url.split(request.url.path).skip(1).toList();
+    if (components.isEmpty) return new shelf.Response.notFound(null);
+
+    var handler = _handlers.remove(components.removeAt(0));
+    if (handler == null) return new shelf.Response.notFound(null);
+    return handler(request);
+  }
+}