Small fix to perf.dart.

pkg/front_end/lib/kernel_generator.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 /// Defines the front-end API for converting source code to Dart Kernel objects.
 library front_end.kernel_generator;
 
 import 'compilation_error.dart';
 import 'compiler_options.dart';
 import 'dart:async';
@@ skipping 165 matching lines @@
 void _reportErrors(List errors, ErrorHandler onError) {
   if (onError == null) return;
   for (var error in errors) {
     onError(new _DartkError(error));
   }
 }
 
 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);
 }
 
 // TODO(sigmund): delete this class. Dartk should not format errors itself, we
 // should just pass them along.
 class _DartkError implements CompilationError {
   String get correction => null;
   SourceSpan get span => null;
   final String message;
   _DartkError(this.message);
 }