Migrate status file entries when migrating files. Yay!

tools/migration/lib/src/editable_status_file.dart

Index: tools/migration/lib/src/editable_status_file.dart
diff --git a/tools/migration/lib/src/editable_status_file.dart b/tools/migration/lib/src/editable_status_file.dart
new file mode 100644
index 0000000000000000000000000000000000000000..7a3795266da73de500e6aa935e8105b866912f84
--- /dev/null
+++ b/tools/migration/lib/src/editable_status_file.dart
@@ -0,0 +1,109 @@
+// 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:io';
+
+import 'package:status_file/status_file.dart';
+
+import 'io.dart';
+import 'log.dart';
+
+// TODO(rnystrom): This file is kind of a hack. The problem is that a parsed
+// StatusFile doesn't contain all of the original information in the source
+// text. Comments and whitespace are lost. That means a StatusFile itself
+// couldn't be modified and saved back to disc.
+//
+// Instead, this wraps a StatusFile and also lets you modify entire lines in
+// it, including their comments. Ideally, StatusFile would include parsed
+// comments and we would canonicalize other whitespace so that you could
+// modify a StatusFile object then save it back to disc.
+class EditableStatusFile {
+  final String path;
+
+  StatusFile _statusFile;
+  StatusFile get statusFile {
+    if (_statusFile == null) {
+      _statusFile = new StatusFile.read(path);
+    }
+
+    return _statusFile;
+  }
+
+  List<String> _lines;
+
+  EditableStatusFile(this.path);
+
+  /// Gets the line at the given one-based index.
+  String lineAt(int line) {
+    _ensureLines();
+    return _lines[line - 1];
+  }
+
+  /// Delete the numbered [lines] from the file.
+  void delete(List<int> lines) {
+    if (lines.isEmpty) return;
+
+    if (dryRun) {
+      print("Delete lines ${lines.join(', ')} from $path.");
+      return;
+    }
+
+    _ensureLines();
+
+    var deleted = 0;
+    for (var line in lines) {
+      // Adjust index because previous lines have already been removed, shifting
+      // the subsequent line numbers.
+      _lines.removeAt(line - deleted);
+      deleted++;
+    }
+
+    _save();
+  }
+
+  /// Insert [entries] at [line] in the file.
+  void insert(int line, List<String> entries) {
+    if (dryRun) {
+      print("Insert ${bold(path)}, insert at line $line:");
+      entries.forEach(print);
+      return;
+    }
+
+    _ensureLines();
+    _lines.insertAll(line, entries);
+    _save();
+  }
+
+  /// Adds [header] followed by [entries] to the end of the file.
+  void append(String header, List<String> entries) {
+    if (dryRun) {
+      print("To ${bold(path)}, append:");
+      print(header);
+      entries.forEach(print);
+      return;
+    }
+
+    _ensureLines();
+    _lines.add("");
+
+    _lines.add(header);
+    _lines.addAll(entries);
+    _save();
+  }
+
+  void _ensureLines() {
+    if (_lines == null) {
+      _lines = new File(path).readAsLinesSync();
+    }
+  }
+
+  void _save() {
+    new File(path).writeAsStringSync(_lines.join("\n") + "\n");
+
+    // It needs to be reparsed now since the lines have changed.
+    // TODO(rnystrom): This is kind of hacky and slow, but it gets the job done.
+    _statusFile = null;
+    _lines = null;
+  }
+}