Use URIs rather than paths in front end API.

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

Index: pkg/front_end/test/src/base/uri_resolver_test.dart
diff --git a/pkg/front_end/test/src/base/uri_resolver_test.dart b/pkg/front_end/test/src/base/uri_resolver_test.dart
index 1273a1934480421b775138c019f7a2d089a8f7e0..af21b9e284ea672c376ca02658b3500b0df6d57d 100644
--- a/pkg/front_end/test/src/base/uri_resolver_test.dart
+++ b/pkg/front_end/test/src/base/uri_resolver_test.dart
@@ -21,7 +21,7 @@ abstract class UriResolverTest {
   p.Context get pathContext;
 
   void test_badScheme() {
-    _expectResolution('foo:bar/baz.dart', null);
+    _expectResolutionUri('foo:bar/baz.dart', Uri.parse('foo:bar/baz.dart'));
   }
 
   void test_dart() {
@@ -54,11 +54,11 @@ abstract class UriResolverTest {
   }
 
   void test_noSchemeAbsolute() {
-    _expectResolution('/foo.dart', null);
+    _expectResolutionUri('/foo.dart', Uri.parse('/foo.dart'));
   }
 
   void test_noSchemeRelative() {
-    _expectResolution('foo.dart', null);
+    _expectResolution('foo.dart', 'foo.dart');
   }
 
   void test_package() {
@@ -102,6 +102,13 @@ abstract class UriResolverTest {
   /// Verifies that the resolution of [uriString] produces the path
   /// [expectedResult].
   void _expectResolution(String uriString, String expectedResult) {
+    _expectResolutionUri(uriString,
+        expectedResult == null ? null : pathContext.toUri(expectedResult));
+  }
+
+  /// Verifies that the resolution of [uriString] produces the URI
+  /// [expectedResult].
+  void _expectResolutionUri(String uriString, Uri expectedResult) {
     var packages = {
       'foo': _u('packages/foo/lib/'),
       'bar': _u('packages/bar/lib/')