Temporarily allow to import dart:io in client apps.

tests/compiler/dart2js_extra/unconditional_dartio_import_test.dart

+// Copyright (c) 2017, 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.
+
+
+/// Tests that dart2js allows to import dart:io for web clients, but it
+/// continues to indicate that it is not supported (so config-specific imports
+/// continue to have the same semantics as before).
+library unconditional_dartio_import_test;
+
+import 'dart:io' as io; // import is allowed!
+import 'package:expect/expect.dart';
+
+main() {
+  // their APIs throw:
+  Expect.throws(() => new io.File('name').existsSync());
+
+  // ... but environment variable will indicate it is not supported.
+  Expect.isFalse(const bool.fromEnvironment('dart.library.io'));
+}