Files are added with absolute URIs, but invalidated with file URIs.

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 accbdcf42f4ba6979150f2cde770188c12ae8fc6..ac7e14ad1e310d7bbfb5e5f542f8d82bc971c059 100644
--- a/pkg/front_end/lib/src/incremental/file_state.dart
+++ b/pkg/front_end/lib/src/incremental/file_state.dart
@@ -262,6 +262,10 @@ class FileSystemState {
     }
     return file;
   }
+
+  /// Return the [FileState] for the given [fileUri], or `null` if the
+  /// [fileUri] does not yet correspond to any referenced [FileState].
+  FileState getFileByFileUri(Uri fileUri) => _fileUriToFile[fileUri];
 }
 
 /// List of libraries that reference each other, so form a cycle.