Load web tests using the plugin infrastructure.

lib/src/runner.dart

Index: lib/src/runner.dart
diff --git a/lib/src/runner.dart b/lib/src/runner.dart
index d21868ac4771cfe332c2611730f786256c6a02e3..80f19b3a31dc0aab95865ae327217d6c6eeb0e2b 100644
--- a/lib/src/runner.dart
+++ b/lib/src/runner.dart
@@ -171,7 +171,7 @@ class Runner {
     // browser tests don't store any state we care about and we want them to
     // shut down without waiting for their tear-downs.
     await Future.wait([
-      _loader.closeBrowsers(),
+      _loader.closeEphemeral(),
       _engine.close()
     ]);
     if (timer != null) timer.cancel();