Add a platform plugin infrastructure.

lib/src/runner/environment.dart

Index: lib/src/runner/environment.dart
diff --git a/lib/src/runner/environment.dart b/lib/src/runner/environment.dart
index 9a7053bb6e6c7a49b11ffbb9b900eb1788fec9fd..694f9382bba8fa2d683e8223d1a50db1535b155f 100644
--- a/lib/src/runner/environment.dart
+++ b/lib/src/runner/environment.dart
@@ -7,6 +7,9 @@ import 'package:async/async.dart';
 /// The abstract class of environments in which test suites are
 /// loaded—specifically, browsers and the Dart VM.
 abstract class Environment {
+  /// Whether this environment supports interactive debugging.
+  bool get supportsDebugging;
+
   /// The URL of the Dart VM Observatory for this environment, or `null` if this
   /// environment doesn't run the Dart VM or the URL couldn't be detected.
   Uri get observatoryUrl;