Add a platform plugin infrastructure.

lib/src/runner/plugin/environment.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 'package:async/async.dart';
+
+import '../environment.dart';
+
+/// The default environment for platform plugins.
+class PluginEnvironment implements Environment {
+  final supportsDebugging = false;
+
+  const PluginEnvironment();
+
+  Uri get observatoryUrl => throw new UnsupportedError(
+      "PluginEnvironment.observatoryUrl is not supported.");
+
+  Uri get remoteDebuggerUrl => throw new UnsupportedError(
+      "PluginEnvironment.remoteDebuggerUrl is not supported.");
+
+  CancelableOperation displayPause() =>
+      throw new UnsupportedError(
+          "PluginEnvironment.displayPause is not supported.");
+}