Small fix to perf.dart.

pkg/front_end/tool/perf.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.
 
 /// An entrypoint used to run portions of front_end and measure its performance.
 library front_end.tool.perf;
 
 import 'dart:async';
 import 'dart:io' show exit;
 
@@ skipping 97 matching lines @@
     {bool useSdkSummary: false, bool compileSdk: true}) async {
   // TODO(sigmund): this is here only to compute the input size,
   // we should extract the input size from the frontend instead.
   scanReachableFiles(entryUri);
 
   var dartkTimer = new Stopwatch()..start();
   // TODO(sigmund): add a constructor with named args to compiler options.
   var options = new CompilerOptions()
     ..strongMode = false
     ..compileSdk = compileSdk
-    ..packagesFileUri = new Uri.file('.packages')
+    ..packagesFileUri = Uri.base.resolve('.packages')
     ..onError = ((e) => print('${e.message}'));
   if (useSdkSummary) {
     // TODO(sigmund): adjust path based on the benchmark runner architecture.
     // Possibly let the runner make the file available at an architecture
     // independent location.
     options.sdkSummary =
-        new Uri.file('out/ReleaseX64/dart-sdk/lib/_internal/spec.sum');
+        Uri.base.resolve('out/ReleaseX64/dart-sdk/lib/_internal/spec.sum');
   } else {
-    options.sdkRoot = new Uri.file('sdk');
+    options.sdkRoot = Uri.base.resolve('sdk');
   }
   Program program = await kernelForProgram(entryUri, options);
   dartkTimer.stop();
   var suffix = useSdkSummary ? '_sum' : '';
   report('kernel_gen_e2e${suffix}', dartkTimer.elapsedMicroseconds);
   return program;
 }
 
 /// Generates unlinkmed summaries for all files in [files], and returns them in
 /// an [_UnlinkedSummaries] container.
@@ skipping 202 matching lines @@
 class _Scanner extends Scanner {
   _Scanner(String contents) : super(new CharSequenceReader(contents)) {
     preserveComments = false;
   }
 
   @override
   void reportError(errorCode, int offset, List<Object> arguments) {
     // ignore errors.
   }
 }