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

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

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:typed_data';
 
 import 'package:crypto/crypto.dart';
 import 'package:front_end/file_system.dart';
 import 'package:front_end/src/base/resolve_relative_uri.dart';
@@ skipping 244 matching lines @@
     if (file == null) {
       file = new FileState._(this, absoluteUri, fileUri);
       _uriToFile[absoluteUri] = file;
       _fileUriToFile[fileUri] = file;
 
       // Build the sub-graph of the file.
       await file.refresh();
     }
     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.
 class LibraryCycle {
   final List<FileState> libraries = <FileState>[];
 
   bool get _isForVm {
     return libraries.any((l) => l.uri.toString().endsWith('dart:_vmservice'));
   }
 
@@ skipping 119 matching lines @@
       node.isEvaluated = true;
       cycle.libraries.add(node.file);
     }
     topologicallySortedCycles.add(cycle);
   }
 
   _LibraryNode getNode(FileState file) {
     return nodesOfFiles.putIfAbsent(file, () => new _LibraryNode(this, file));
   }
 }