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

tools/migration/lib/src/fork.dart

Index: tools/migration/lib/src/fork.dart
diff --git a/tools/migration/lib/src/fork.dart b/tools/migration/lib/src/fork.dart
new file mode 100644
index 0000000000000000000000000000000000000000..b5087a05ac4e034a74396197828d4c25d2569a5a
--- /dev/null
+++ b/tools/migration/lib/src/fork.dart
@@ -0,0 +1,117 @@
+// 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;
+
+import 'io.dart';
+import 'log.dart';
+import 'validate.dart';
+
+/// Tracks one test and the various forked locations where it may appear.
+///
+/// * "One" refers to the original Dart 1.0 location of the test: language/,
+///   corelib/, etc.
+///
+/// * "Strong" is the DDC fork of the file: language_strong, etc.
+///
+/// * "Two" is the migrated 2.0 destination location: language_2, etc.
+class Fork {
+  final String twoPath;
+  String onePath;
+  String strongPath;
+
+  String get twoSource {
+    if (twoPath == null) return null;
+    if (_twoSource == null) _twoSource = readFile(twoPath);
+    return _twoSource;
+  }
+
+  String _twoSource;
+
+  String get oneSource {
+    if (onePath == null) return null;
+    if (_oneSource == null) _oneSource = readFile(onePath);
+    return _oneSource;
+  }
+
+  String _oneSource;
+
+  String get strongSource {
+    if (strongPath == null) return null;
+    if (_strongSource == null) _strongSource = readFile(strongPath);
+    return _strongSource;
+  }
+
+  bool get isMigrated => new File(p.join(testRoot, twoPath)).existsSync();
+
+  String _strongSource;
+
+  Fork(this.twoPath);
+
+  List<String> migrate() {
+    print("- ${bold(twoPath)}:");
+
+    var todos = <String>[];
+
+    if (onePath == null && strongPath == null) {
+      // It's already been migrated, so there's nothing to move.
+      note("Is already migrated.");
+    } else if (isMigrated) {
+      // If there is a migrated version and it's the same as an unmigrated one,
+      // delete the unmigrated one.
+      if (onePath != null) {
+        if (oneSource == twoSource) {
+          deleteFile(onePath);
+          done("Deleted already-migrated $onePath.");
+        } else {
+          note("${bold(onePath)} does not match already-migrated file.");
+          todos.add("Merge from ${bold(onePath)} into this file.");
+          validateFile(onePath, oneSource);
+        }
+      }
+
+      if (strongPath != null) {
+        if (strongSource == twoSource) {
+          deleteFile(strongPath);
+          done("Deleted already-migrated ${bold(strongPath)}.");
+        } else {
+          note("${bold(strongPath)} does not match already-migrated file.");
+          todos.add("Merge from ${bold(strongPath)} into this file.");
+          validateFile(strongPath, strongSource);
+        }
+      }
+    } else {
+      if (strongPath == null) {
+        // If it only exists in one place, just move it.
+        moveFile(onePath, twoPath);
+        done("Moved from ${bold(onePath)} (no strong mode fork).");
+      } else if (onePath == null) {
+        // If it only exists in one place, just move it.
+        moveFile(strongPath, twoPath);
+        done("Moved from ${bold(strongPath)} (no 1.0 mode fork).");
+      } else if (oneSource == strongSource) {
+        // The forks are identical, pick one.
+        moveFile(onePath, twoPath);
+        deleteFile(strongPath);
+        done("Merged identical forks.");
+        validateFile(twoPath, oneSource);
+      } else {
+        // Otherwise, a manual merge is required. Start with the strong one.
+        moveFile(strongPath, twoPath);
+        done("Moved strong fork, kept 1.0 fork, manual merge required.");
+        todos.add("Merge from ${bold(onePath)} into this file.");
+        validateFile(onePath, oneSource);
+      }
+    }
+
+    // See what work is left to be done in the migrated file.
+    if (isMigrated) {
+      validateFile(twoPath, twoSource, todos);
+    }
+
+    return todos;
+  }
+}