Handle migration status entries into split up status files.

tools/migration/lib/src/editable_status_file.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: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);
+      if (new File(path).existsSync()) {
+        _statusFile = new StatusFile.read(path);
+      } else {
+        _statusFile = new StatusFile(path);
+      }
     }
 
     return _statusFile;
   }
 
   List<String> _lines;
 
   EditableStatusFile(this.path);
 
   /// Gets the line at the given one-based index.
@@ skipping 47 matching lines @@
     }
 
     _ensureLines();
     _lines.add("");
 
     _lines.add(header);
     _lines.addAll(entries);
     _save();
   }
 
   void _ensureLines() {

[jcollins, 2017/08/04 16:55:56]

optional: replace this and _lines with a getter for lines that ensures that we
load the _lines in if we haven't already, so that callers don't have to do that.

[Bob Nystrom, 2017/08/04 17:33:01]

Good idea, done.

     if (_lines == null) {
-      _lines = new File(path).readAsLinesSync();
+      var file = new File(path);
+      if (file.existsSync()) {
+        _lines = file.readAsLinesSync();
+      } else {
+        _lines = [
+          "# 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."
+        ];
+      }
     }
   }
 
   void _save() {
+    if (dryRun) {
+      print("Save ${bold(path)}");
+      return;
+    }
+
     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;
   }
 }