Add an Environment class that's exposed through RunnerSuite.

lib/src/runner/vm/environment.dart

Index: lib/src/runner/vm/environment.dart
diff --git a/bin/test.dart b/lib/src/runner/vm/environment.dart
similarity index 50%
copy from bin/test.dart
copy to lib/src/runner/vm/environment.dart
index 0b40282a606bc05d57acede2d8ae0eaa798a1e43..9833d81806d1fdddeb9a9166ee684d1c0320402d 100644
--- a/bin/test.dart
+++ b/lib/src/runner/vm/environment.dart
@@ -2,4 +2,11 @@
 // 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.vm.environment;
+
+import '../environment.dart';
+
+/// The environment for test suites loaded in an isolate in this Dart VM.
+class VMEnvironment implements Environment {
+  const VMEnvironment();
+}