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

pkg/front_end/lib/src/fasta/scanner/token.dart

 // Copyright (c) 2011, 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.
 
 library fasta.scanner.token;
 
 import '../../scanner/token.dart' as analyzer;
 import '../../scanner/token.dart' show TokenType;
 
+import 'package:kernel/ast.dart' show TreeNode;

[ahe, 2017/04/25 17:39:59]

It feels a bit much to import package:kernel here to get a symbolic name for -1.
Could we cheat and use -1?

Another alternative is to move offsetForToken to ../scanner.dart. That file also
exports Token, so perhaps you can remove the change to subpackage rules?

[Paul Berry, 2017/04/25 22:00:55]

What's bothering you, the import of kernel or the change to the subpackage
rules?  Cheating and using -1 would fix the former (at the expense of some
future risk if TreeNode.noOffset is ever changed) but not the latter.  Moving
offsetForToken would avoid the need to change the subpackage rules, but wouldn't
address the import of kernel (it would just be a different file that has to
import kernel).

I like using TreeNode.noOffset--it feels nice to be explicit about the fact that
we're using -1 in order to agree with kernel conventions.  But I agree that it
seems weird and unnecessary for the scanner to have to depend on kernel.  Maybe
a better approach would be to move offsetForToken somewhere inside
lib/src/fasta/kernel (e.g. lib/src/fasta/kernel/utils.dart).  That would be less
weird, since nobody would be surprised to see a file in lib/src/fasta/kernel
importing kernel.  And it would avoid the need to change the subpackage rules
too, since all of the callers of offsetForToken are in lib/src/fasta/kernel.

I'll try that.

[ahe, 2017/04/26 08:34:36]

Both, I think.
OK

+
 import 'precedence.dart'
     show AS_INFO, BAD_INPUT_INFO, EOF_INFO, IS_INFO, KEYWORD_INFO;
 
 import 'token_constants.dart' show IDENTIFIER_TOKEN;
 
 import 'string_canonicalizer.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;
+
 /**
  * A token that doubles as a linked list.
  */
 abstract class Token implements analyzer.TokenWithComment {
   /**
    * The character offset of the start of this token within the source text.
    */
   int charOffset;
 
   Token(this.charOffset);
@@ skipping 656 matching lines @@
       identical(value, "<=") ||
       identical(value, "<") ||
       identical(value, "&") ||
       identical(value, "^") ||
       identical(value, "|");
 }
 
 bool isTernaryOperator(String value) => identical(value, "[]=");
 
 bool isMinusOperator(String value) => identical(value, "-");