Use URIs rather than paths in front end API.

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 110 matching lines @@
   return program;
 }
 
 /// Create a [DartLoader] using the provided [options].
 ///
 /// If [options] contain no configuration to resolve `.packages`, the [entry]
 /// file will be used to search for a `.packages` file.
 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);
 }
 
 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);
 }
 
 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');
+  }
+  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);
 }