Handle migration status entries into split up status files.

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
index 7a3795266da73de500e6aa935e8105b866912f84..ad5c04537921af7174646d60284efe537ef0e28b 100644
--- a/tools/migration/lib/src/editable_status_file.dart
+++ b/tools/migration/lib/src/editable_status_file.dart
@@ -24,7 +24,11 @@ class EditableStatusFile {
   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;
@@ -94,11 +98,27 @@ class EditableStatusFile {
 
   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.