Implement GC for FileState(s).

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 123 matching lines @@
 foo() {
   print(22);
 }
 bar() {
   print(33);
 }
 baz() => 44;
 ''');
   }
 
+  test_gc() async {
+    var a = writeFile('/a.dart', '');
+    var b = writeFile('/b.dart', '');
+    var c = writeFile('/c.dart', 'import "a.dart";');
+    var d = writeFile('/d.dart', 'import "b.dart";');
+    var e = writeFile(
+        '/e.dart',
+        r'''
+import "c.dart";
+import "d.dart";
+''');
+
+    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.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);
+    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', '');
     var c = writeFile('/c.dart', '');
     var d = writeFile(
         '/d.dart',
         r'''
 import "a.dart";
 export "b.dart";
 part "c.dart";
@@ skipping 189 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')}
 ''');
   }
 }