Cache transitive files in FileState.

pkg/front_end/test/src/incremental/file_state_test.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 'package:front_end/memory_file_system.dart';
 import 'package:front_end/src/fasta/translate_uri.dart';
 import 'package:front_end/src/incremental/byte_store.dart';
 import 'package:front_end/src/incremental/file_state.dart';
@@ skipping 473 matching lines @@
     FileState b = await fsState.getFile(bUri);
     FileState c = await fsState.getFile(cUri);
 
     List<LibraryCycle> order = c.topologicalOrder;
     expect(order, hasLength(3));
     expect(order[0].libraries, contains(core));
     expect(order[1].libraries, unorderedEquals([a]));
     expect(order[2].libraries, unorderedEquals([b, c]));
   }
 
+  test_transitiveFiles() async {
+    var a = writeFile('/a.dart', "");
+    var b = writeFile('/b.dart', "");
+    var c = writeFile('/c.dart', "import 'b.dart';");
+
+    FileState aFile = await fsState.getFile(a);
+    FileState bFile = await fsState.getFile(b);
+    FileState cFile = await fsState.getFile(c);
+
+    // Only c.dart and b.dart are in the transitive closure.
+    expect(cFile.transitiveFiles, contains(cFile));
+    expect(cFile.transitiveFiles, contains(bFile));
+    expect(cFile.transitiveFiles, isNot(contains(aFile)));
+    expect(bFile.transitiveFiles, isNot(contains(aFile)));
+
+    // Import a.dart into b.dart, changes c.dart transitive closure.
+    writeFile('/b.dart', "import 'a.dart';");
+    await bFile.refresh();
+    expect(cFile.transitiveFiles, contains(cFile));
+    expect(cFile.transitiveFiles, contains(bFile));
+    expect(cFile.transitiveFiles, contains(aFile));
+    expect(bFile.transitiveFiles, contains(aFile));
+
+    // Stop importing a.dart into b.dart, changes c.dart transitive closure.
+    writeFile('/b.dart', "");
+    await bFile.refresh();
+    expect(cFile.transitiveFiles, contains(cFile));
+    expect(cFile.transitiveFiles, contains(bFile));
+    expect(cFile.transitiveFiles, isNot(contains(aFile)));
+    expect(bFile.transitiveFiles, isNot(contains(aFile)));
+  }
+
   /// Write the given [text] of the file with the given [path] into the
   /// virtual filesystem.  Return the URI of the file.
   Uri writeFile(String path, String text) {
     Uri uri = Uri.parse('file://$path');
     fileSystem.entityForUri(uri).writeAsStringSync(text);
     return uri;
   }
 
   void _assertImportedUris(FileState file, List<Uri> expectedUris) {
     Iterable<Uri> importedUris = _toUris(file.importedLibraries);
     expect(importedUris, unorderedEquals(expectedUris));
   }
 
   Iterable<Uri> _toUris(List<FileState> files) {
     return files.map((f) => f.uri);
   }
 
   Uri _writeFileDirectives(String path,
       {List<String> imports: const [], List<String> exports: const []}) {
     return writeFile(
         path,
         '''
 ${imports.map((uri) => 'import "$uri";').join('\n')}
 ${exports.map((uri) => 'export "$uri";').join('\n')}
 ''');
   }
 }