Small fix to perf.dart.

pkg/front_end/lib/kernel_generator.dart

Index: pkg/front_end/lib/kernel_generator.dart
diff --git a/pkg/front_end/lib/kernel_generator.dart b/pkg/front_end/lib/kernel_generator.dart
index 0ae2fb3c232c3972cb42593340d31d6670f90884..086f45adea80856cf9d65959acb630cde6db9b6b 100644
--- a/pkg/front_end/lib/kernel_generator.dart
+++ b/pkg/front_end/lib/kernel_generator.dart
@@ -183,7 +183,7 @@ void _reportErrors(List errors, ErrorHandler onError) {
 String _uriToPath(Uri uri, CompilerOptions options) {
   if (uri == null) return null;
   if (uri.scheme != 'file') {
-    throw new StateError('Only file URIs are supported');
+    throw new StateError('Only file URIs are supported: $uri');
   }
   return options.fileSystem.context.fromUri(uri);
 }