Use URIs rather than paths in front end API.

pkg/front_end/tool/perf.dart

Index: pkg/front_end/tool/perf.dart
diff --git a/pkg/front_end/tool/perf.dart b/pkg/front_end/tool/perf.dart
index a69251bb81889a0a93b435c8c4232ca12d007e8d..ebe34e34855a2fe1da37dfd1f4329a80ae93e94b 100644
--- a/pkg/front_end/tool/perf.dart
+++ b/pkg/front_end/tool/perf.dart
@@ -115,15 +115,16 @@ Future<Program> generateKernel(Uri entryUri,
   var options = new CompilerOptions()
     ..strongMode = false
     ..compileSdk = compileSdk
-    ..packagesFilePath = '.packages'
+    ..packagesFilePath = new Uri.file('.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 = 'out/ReleaseX64/dart-sdk/lib/_internal/spec.sum';
+    options.sdkSummary =
+        new Uri.file('out/ReleaseX64/dart-sdk/lib/_internal/spec.sum');
   } else {
-    options.sdkPath = 'sdk';
+    options.sdkPath = new Uri.file('sdk');
   }
   Program program = await kernelForProgram(entryUri, options);
   dartkTimer.stop();