Use URIs rather than paths in front end API.

pkg/front_end/tool/example.dart

Index: pkg/front_end/tool/example.dart
diff --git a/pkg/front_end/tool/example.dart b/pkg/front_end/tool/example.dart
index 1d49b228759959446c8f8ffb560b6f9a4747ed5a..a637a7ffad9dbafa76843feb6d820e30e135edcc 100644
--- a/pkg/front_end/tool/example.dart
+++ b/pkg/front_end/tool/example.dart
@@ -13,8 +13,8 @@ Future dumpToSink(Program program, StreamSink<List<int>> sink) {
 Future kernelToSink(Uri entry, StreamSink<List<int>> sink) async {
   var program = await kernelForProgram(entry,
       new CompilerOptions()
-        ..sdkPath = 'sdk'
-        ..packagesFilePath = '.packages'
+        ..sdkPath = new Uri.file('sdk')
+        ..packagesFilePath = new Uri.file('.packages')
         ..onError = (e) => print(e.message));
 
   await dumpToSink(program, sink);