Add a hack to globally register a platform plugin.

lib/src/runner/plugin/hack_register_platform.dart

+// Copyright (c) 2016, 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.
+
+import 'dart:collection';
+
+import '../../backend/test_platform.dart';
+import '../../utils.dart';
+import 'platform.dart';
+
+/// The functions to use to load [_platformPlugins] in all loaders.
+///
+/// **Do not access this outside the test package**.
+final platformCallbacks =
+    new UnmodifiableMapView<TestPlatform, AsyncFunction>(_platformCallbacks);
+final _platformCallbacks = <TestPlatform, AsyncFunction>{};
+
+/// **Do not call this function without express permission from the test package
+/// authors**.
+///
+/// Registers a [PlatformPlugin] for [platforms].
+///
+/// This globally registers a plugin for all [Loader]s. When the runner first
+/// requests that a suite be loaded for one of the given platforms, this will
+/// call [getPlugin] to load the platform plugin. It may return either a
+/// [PlatformPlugin] or a [Future<PlatformPlugin>]. That plugin is then
+/// preserved and used to load all suites for all matching platforms.
+///
+/// This overwrites the default plugins for those platforms.
+void registerPlatformPlugin(Iterable<TestPlatform> platforms, getPlugin()) {

[kevmoo, 2016/02/23 22:54:50]

Feels weird that getPlugin() doesn't take in an arg – but I may be missing
something

[nweiz, 2016/02/24 00:21:22]

What arg do you expect it to take in?

[kevmoo, 2016/02/24 00:28:54]

The platform? – although I guess you can call this many times.

Never mind. :-)

+  for (var platform in platforms) {
+    _platformCallbacks[platform] = getPlugin;
+  }
+}