Use _provider.pathContext to convert URIs to files.

pkg/analyzer/lib/file_system/file_system.dart

Index: pkg/analyzer/lib/file_system/file_system.dart
diff --git a/pkg/analyzer/lib/file_system/file_system.dart b/pkg/analyzer/lib/file_system/file_system.dart
index 58da59b1257413046b737d80986ea2bd27bf4113..5ef0196c56ca176b677724b7ebd42dae76a321fc 100644
--- a/pkg/analyzer/lib/file_system/file_system.dart
+++ b/pkg/analyzer/lib/file_system/file_system.dart
@@ -130,7 +130,8 @@ class ResourceUriResolver extends UriResolver {
     if (!_isFileUri(uri)) {
       return null;
     }
-    Resource resource = _provider.getResource(fromUri(uri));
+    Resource resource = _provider.getResource(_provider.pathContext.fromUri(uri)
+        );
     if (resource is File) {
       return resource.createSource(uri);
     }