Better support for re-running in the middle of a migration.

tools/migration/lib/src/io.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:path/path.dart' as p;
+
+/// True if the file system should be left untouched.
+bool dryRun = false;
+
+final String sdkRoot =
+    p.normalize(p.join(p.dirname(p.fromUri(Platform.script)), '../../../'));
+
+final String testRoot = p.join(sdkRoot, "tests");
+
+/// Moves the file from [from] to [to], which are both assumed to be relative
+/// paths inside "tests".
+void moveFile(String from, String to) {
+  if (dryRun) {
+    print("  Dry run: move $from to $to");
+    return;
+  }
+
+  // Create the directory if needed.
+  new Directory(p.dirname(p.join(testRoot, to))).createSync(recursive: true);
+
+  new File(p.join(testRoot, from)).renameSync(p.join(testRoot, to));
+}
+
+/// Reads the contents of the file at [path], which is assumed to be relative
+/// within "tests".
+String readFile(String path) {
+  return new File(p.join(testRoot, path)).readAsStringSync();
+}
+
+/// Deletes the file at [path], which is assumed to be relative within "tests".
+void deleteFile(String path) {
+  if (dryRun) {
+    print("  Dry run: delete $path");
+    return;
+  }
+
+  new File(p.join(testRoot, path)).deleteSync();
+}
+
+/// Returns a list of the paths to all files within [dir], which is
+/// assumed to be relative to the SDK's "tests" directory and having file
+/// [extension].
+Iterable<String> listFiles(String dir, {String extension = ".dart"}) {
+  return new Directory(p.join(testRoot, dir))
+      .listSync(recursive: true)
+      .map((entry) {
+    if (!entry.path.endsWith(extension)) return null;
+
+    return entry.path;
+  }).where((path) => path != null);
+}