Change accessors and AstFactory to use tokens rather than file offsets.

pkg/front_end/lib/src/fasta/kernel/utils.dart

Index: pkg/front_end/lib/src/fasta/kernel/utils.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/utils.dart b/pkg/front_end/lib/src/fasta/kernel/utils.dart
new file mode 100644
index 0000000000000000000000000000000000000000..0b5f3f497bf45bd31854ea3e07f32c69432e94d1
--- /dev/null
+++ b/pkg/front_end/lib/src/fasta/kernel/utils.dart
@@ -0,0 +1,11 @@
+// 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 'package:front_end/src/fasta/scanner/token.dart';
+import 'package:kernel/ast.dart';
+
+/// A null-aware alternative to `token.offset`.  If [token] is `null`, returns
+/// `TreeNode.noOffset`.
+int offsetForToken(Token token) =>
+    token == null ? TreeNode.noOffset : token.offset;