Add the test for updating a part.

pkg/front_end/lib/src/incremental/file_state.dart

Index: pkg/front_end/lib/src/incremental/file_state.dart
diff --git a/pkg/front_end/lib/src/incremental/file_state.dart b/pkg/front_end/lib/src/incremental/file_state.dart
index 4342b357f562cbcb3e60bc29643f2321b1dc461d..c1bcfeb17b0ff2dc245f361a5f7f13aff77f5ead 100644
--- a/pkg/front_end/lib/src/incremental/file_state.dart
+++ b/pkg/front_end/lib/src/incremental/file_state.dart
@@ -123,8 +123,13 @@ class FileState {
     }
 
     // Resolve the absolute URI into the absolute file URI.
-    Uri resolvedUri = _fsState.uriTranslator.translate(absoluteUri);
-    if (resolvedUri == null) return;
+    Uri resolvedUri;
+    if (absoluteUri.isScheme('file')) {
+      resolvedUri = absoluteUri;
+    } else {
+      resolvedUri = _fsState.uriTranslator.translate(absoluteUri);
+      if (resolvedUri == null) return;
+    }
 
     FileState file = await _fsState.getFile(resolvedUri);
     files.add(file);