Report unused files to watch.

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/file_state.dart';
 import 'package:test/test.dart';
@@ skipping 145 matching lines @@
     var eFile = await fsState.getFile(e);
 
     // The root and four files.
     expect(fsState.fileUris, contains(e));
     expect(fsState.fileUris, contains(a));
     expect(fsState.fileUris, contains(b));
     expect(fsState.fileUris, contains(c));
     expect(fsState.fileUris, contains(d));
 
     // No changes after GC.
-    fsState.gc(e);
+    expect(fsState.gc(e), isEmpty);
     expect(fsState.fileUris, contains(e));
     expect(fsState.fileUris, contains(a));
     expect(fsState.fileUris, contains(b));
     expect(fsState.fileUris, contains(c));
     expect(fsState.fileUris, contains(d));
 
     // Update e.dart so that it does not reference c.dart anymore.
     // Then GC removes both c.dart and a.dart it references.
     writeFile(
         '/e.dart',
         r'''
 import "d.dart";
 ''');
     await eFile.refresh();
-    fsState.gc(e);
+    {
+      var gcFiles = fsState.gc(e);
+      expect(gcFiles.map((file) => file.uri), unorderedEquals([a, c]));
+    }
     expect(fsState.fileUris, contains(e));
     expect(fsState.fileUris, isNot(contains(a)));
     expect(fsState.fileUris, contains(b));
     expect(fsState.fileUris, isNot(contains(c)));
     expect(fsState.fileUris, contains(d));
   }
 
   test_getFile() async {
     var a = writeFile('/a.dart', '');
     var b = writeFile('/b.dart', '');
@@ skipping 196 matching lines @@
   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')}
 ''');
   }
 }