Add a platform plugin infrastructure.

lib/src/runner/vm/environment.dart

-// 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.
-
-import 'package:async/async.dart';
-
-import '../environment.dart';
-
-/// The environment for test suites loaded in an isolate in this Dart VM.
-class VMEnvironment implements Environment {
-  const VMEnvironment();
-
-  Uri get observatoryUrl => throw new UnsupportedError(
-      "VMEnvironment.observatoryUrl is not currently supported.");
-
-  Uri get remoteDebuggerUrl => throw new UnsupportedError(
-      "VMEnvironment.observatoryUrl is not supported.");
-
-  CancelableOperation displayPause() =>
-      throw new UnsupportedError(
-          "The VM doesn't yet support Environment.displayPause.");
-}