Use URIs rather than paths in front end API.

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 a87bb683b44dc71eb28c9cf328fba10469ada4f6..5bde9b3de8267581349ff3ce0ca20bbac2f537e8 100644
--- a/pkg/front_end/lib/kernel_generator.dart
+++ b/pkg/front_end/lib/kernel_generator.dart
@@ -128,7 +128,8 @@ Future<Program> kernelForBuildUnit(
 Future<DartLoader> _createLoader(CompilerOptions options,
     {Repository repository, Uri entry}) async {
   var kernelOptions = _convertOptions(options);
-  var packages = await createPackages(options.packagesFilePath,
+  var packages = await createPackages(
+      _uriToPath(options.packagesFilePath, options),
       discoveryPath: entry?.path);
   return new DartLoader(
       repository ?? new Repository(), kernelOptions, packages);
@@ -137,11 +138,12 @@ Future<DartLoader> _createLoader(CompilerOptions options,
 DartOptions _convertOptions(CompilerOptions options) {
   return new DartOptions(
       strongMode: options.strongMode,
-      sdk: options.sdkPath,
+      sdk: _uriToPath(options.sdkPath, options),
       // TODO(sigmund): make it possible to use summaries and still compile the
       // sdk sources.
-      sdkSummary: options.compileSdk ? null : options.sdkSummary,
-      packagePath: options.packagesFilePath,
+      sdkSummary:
+          options.compileSdk ? null : _uriToPath(options.sdkSummary, options),
+      packagePath: _uriToPath(options.packagesFilePath, options),
       declaredVariables: options.declaredVariables);
 }
 
@@ -152,6 +154,14 @@ 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');
+  }
+  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 {