Use URIs rather than paths in front end API.

pkg/front_end/test/src/base/processed_options_test.dart

 // Copyright (c) 2017, 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.
 
 import 'package:front_end/compiler_options.dart';
 import 'package:front_end/memory_file_system.dart';
 import 'package:front_end/src/base/processed_options.dart';
 import 'package:path/path.dart' as pathos;
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(ProcessedOptionsTest);
   });
 }
 
 @reflectiveTest
 class ProcessedOptionsTest {
-  final fileSystem = new MemoryFileSystem(pathos.posix, '/');
+  final fileSystem = new MemoryFileSystem(pathos.posix, Uri.parse('file:///'));
 
   test_compileSdk_false() {
     for (var value in [false, true]) {
       var raw = new CompilerOptions()..compileSdk = value;
       var processed = new ProcessedOptions(raw);
       expect(processed.compileSdk, value);
     }
   }
 
   test_fileSystem_noBazelRoots() {
     // When no bazel roots are specified, the filesystem should be passed
     // through unmodified.
     var raw = new CompilerOptions()..fileSystem = fileSystem;
     var processed = new ProcessedOptions(raw);
     expect(processed.fileSystem, same(fileSystem));
   }
 
   test_getUriResolver_explicitPackagesFile() async {
     // This .packages file should be ignored.
-    fileSystem.entityForPath('/.packages').writeAsStringSync('foo:bar\n');
+    fileSystem
+        .entityForUri(Uri.parse('file:///.packages'))
+        .writeAsStringSync('foo:bar\n');
     // This one should be used.
     fileSystem
-        .entityForPath('/explicit.packages')
+        .entityForUri(Uri.parse('file:///explicit.packages'))
         .writeAsStringSync('foo:baz\n');
     var raw = new CompilerOptions()
       ..fileSystem = fileSystem
-      ..packagesFilePath = '/explicit.packages';
+      ..packagesFilePath = Uri.parse('file:///explicit.packages');
     var processed = new ProcessedOptions(raw);
     var uriResolver = await processed.getUriResolver();
     expect(uriResolver.packages, {'foo': Uri.parse('file:///baz/')});
     expect(uriResolver.pathContext, same(fileSystem.context));
   }
 
   test_getUriResolver_explicitPackagesFile_withBaseLocation() async {
     // This .packages file should be ignored.
-    fileSystem.entityForPath('/.packages').writeAsStringSync('foo:bar\n');
+    fileSystem
+        .entityForUri(Uri.parse('file:///.packages'))
+        .writeAsStringSync('foo:bar\n');
     // This one should be used.
     fileSystem
-        .entityForPath('/base/location/explicit.packages')
+        .entityForUri(Uri.parse('file:///base/location/explicit.packages'))
         .writeAsStringSync('foo:baz\n');
     var raw = new CompilerOptions()
       ..fileSystem = fileSystem
-      ..packagesFilePath = '/base/location/explicit.packages';
+      ..packagesFilePath = Uri.parse('file:///base/location/explicit.packages');
     var processed = new ProcessedOptions(raw);
     var uriResolver = await processed.getUriResolver();
     expect(
         uriResolver.packages, {'foo': Uri.parse('file:///base/location/baz/')});
     expect(uriResolver.pathContext, same(fileSystem.context));
   }
 
   test_getUriResolver_noPackages() async {
     // .packages file should be ignored.
-    fileSystem.entityForPath('/.packages').writeAsStringSync('foo:bar\n');
+    fileSystem
+        .entityForUri(Uri.parse('file:///.packages'))
+        .writeAsStringSync('foo:bar\n');
     var raw = new CompilerOptions()
       ..fileSystem = fileSystem
-      ..packagesFilePath = '';
+      ..packagesFilePath = new Uri();
     var processed = new ProcessedOptions(raw);
     var uriResolver = await processed.getUriResolver();
     expect(uriResolver.packages, isEmpty);
     expect(uriResolver.pathContext, same(fileSystem.context));
   }
 }