Add an Environment class that's exposed through RunnerSuite.

lib/src/runner/environment.dart

Index: lib/src/runner/environment.dart
diff --git a/bin/test.dart b/lib/src/runner/environment.dart
similarity index 54%
copy from bin/test.dart
copy to lib/src/runner/environment.dart
index 0b40282a606bc05d57acede2d8ae0eaa798a1e43..5f2b9467ef59f47f65d5307f357fa235d840d900 100644
--- a/bin/test.dart
+++ b/lib/src/runner/environment.dart
@@ -2,4 +2,9 @@
 // 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.
 
-export 'package:test/src/executable.dart';
+library test.runner.environment;
+
+/// The abstract class of environments in which test suites are
+/// loaded—specifically, browsers and the Dart VM.
+abstract class Environment {
+}