Add an Environment class that's exposed through RunnerSuite.

lib/test.dart

Index: lib/test.dart
diff --git a/lib/test.dart b/lib/test.dart
index aa5d4ec449cf9a42a1bf01a40764d0900de4965b..c74ac9a0a434fa7f70da35e8155bce3f4b2b033e 100644
--- a/lib/test.dart
+++ b/lib/test.dart
@@ -14,6 +14,7 @@ import 'src/frontend/timeout.dart';
 import 'src/runner/engine.dart';
 import 'src/runner/reporter/expanded.dart';
 import 'src/runner/runner_suite.dart';
+import 'src/runner/vm/environment.dart';
 import 'src/utils.dart';
 
 export 'package:matcher/matcher.dart';
@@ -51,6 +52,9 @@ Declarer get _declarer {
   _globalDeclarer = new Declarer();
   scheduleMicrotask(() async {
     var suite = new RunnerSuite(
+        // TODO(nweiz): Use a different environment if VM environment starts
+        // import dart:io before cross-platform libraries exist.
+        const VMEnvironment(),
         _globalDeclarer.tests,
         path: p.prettyUri(Uri.base),
         platform: TestPlatform.vm,