Use URIs rather than paths in front end API.

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

Index: pkg/front_end/test/src/base/processed_options_test.dart
diff --git a/pkg/front_end/test/src/base/processed_options_test.dart b/pkg/front_end/test/src/base/processed_options_test.dart
index ff34028afb3a28f5e4016854996fbbc401cbcb11..e12b88c71371863559d82843c71ff8c852d4202e 100644
--- a/pkg/front_end/test/src/base/processed_options_test.dart
+++ b/pkg/front_end/test/src/base/processed_options_test.dart
@@ -17,7 +17,7 @@ main() {
 
 @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]) {
@@ -37,14 +37,16 @@ class ProcessedOptionsTest {
 
   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/')});
@@ -53,14 +55,16 @@ class ProcessedOptionsTest {
 
   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(
@@ -70,10 +74,12 @@ class ProcessedOptionsTest {
 
   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);