Add a script to run the tests in a migration block.

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
index 4f2697e38d9df88c4a7b296b9b0b9a67c467e8aa..84a276b595e3a7f9e320bb161dddfaa6f6f30d54 100644
--- a/tools/migration/lib/src/fork.dart
+++ b/tools/migration/lib/src/fork.dart
@@ -11,6 +11,60 @@ import 'log.dart';
 import 'test_directories.dart';
 import 'validate.dart';
 
+int findFork(List<Fork> forks, String description) {
+  var matches = <int>[];
+
+  for (var i = 0; i < forks.length; i++) {
+    if (forks[i].twoPath.contains(description)) matches.add(i);
+  }
+
+  if (matches.isEmpty) {
+    print('Could not find a test matching "${bold(description)}".');
+    return null;
+  } else if (matches.length == 1) {
+    return matches.first;
+  } else {
+    print('Description "${bold(description)}" is ambiguous. Could be any of:');
+    for (var i in matches) {
+      print("- ${forks[i].twoPath.replaceAll(description, bold(description))}");
+    }
+
+    print("Please use a more precise description.");
+    return null;
+  }
+}
+
+/// Loads all of the unforked test files.
+///
+/// Creates an list of [Fork]s, ordered by their destination paths. Handles
+/// tests that only appear in one fork or the other, or both.
+List<Fork> scanTests() {
+  var tests = <String, Fork>{};
+
+  for (var fromDir in fromRootDirs) {
+    var twoDir = toTwoDirectory(fromDir);
+    for (var path in listFiles(fromDir)) {
+      var fromPath = p.relative(path, from: testRoot);
+      var twoPath = p.join(twoDir, p.relative(fromPath, from: fromDir));
+
+      tests.putIfAbsent(twoPath, () => new Fork(twoPath));
+    }
+  }
+
+  // Include tests that have already been migrated too so we can show what
+  // works remains to be done in them.
+  for (var dir in twoRootDirs) {
+    for (var path in listFiles(dir)) {
+      var twoPath = p.relative(path, from: testRoot);
+      tests.putIfAbsent(twoPath, () => new Fork(twoPath));
+    }
+  }
+
+  var sorted = tests.values.toList();
+  sorted.sort((a, b) => a.twoPath.compareTo(b.twoPath));
+  return sorted;
+}
+
 /// 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/,