update fasta scanner to always generate INDEX or INDEX_EQ tokens

pkg/compiler/lib/src/tree/unparser.dart

 // Copyright (c) 2012, 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.dart' show Token;
 import 'package:front_end/src/fasta/scanner/token_constants.dart' as Tokens
     show IDENTIFIER_TOKEN, KEYWORD_TOKEN, PLUS_TOKEN;
 import '../util/util.dart';
 import 'nodes.dart';
 
@@ skipping 368 matching lines @@
       if (spaces) space();
       visit(link.head);
     }
   }
 
   unparseNodeList(NodeList node, {bool spaces: true}) {
     addToken(node.beginToken);
     if (node.nodes != null) {
       unparseNodeListFrom(node, node.nodes, spaces: spaces);
     }
-    if (node.endToken != null) write(node.endToken.lexeme);
+    // if the NodeList is a single "[]" token

[ahe, 2017/06/01 14:45:01]

if -> If

+    // then beginToken == endToken and only write beginToken.
+    if (node.endToken != null && node.endToken != node.beginToken) {
+      write(node.endToken.lexeme);
+    }
   }
 
   visitNodeList(NodeList node) {
     unparseNodeList(node);
   }
 
   visitOperator(Operator node) {
     visitIdentifier(node);
   }
 
@@ skipping 509 matching lines @@
   }
 
   visitStringNode(StringNode node) {
     throw 'internal error'; // Should not be called.
   }
 
   visitTypeAnnotation(TypeAnnotation node) {
     throw 'internal error'; // Should not be called.
   }
 }