Tighten the span around index operators.

lib/src/source_visitor.dart

 // Copyright (c) 2014, 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 dart_style.src.source_visitor;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/src/generated/source.dart';
 
@@ skipping 1187 matching lines @@
   /// call chains.
   void finishIndexExpression(IndexExpression node) {
     if (node.target is IndexExpression) {
       // Edge case: On a chain of [] accesses, allow splitting between them.
       // Produces nicer output in cases like:
       //
       //     someJson['property']['property']['property']['property']...
       soloZeroSplit();
     }
 
-    builder.startSpan();
+    builder.startSpan(Cost.index);
     token(node.leftBracket);
     soloZeroSplit();
     visit(node.index);
     token(node.rightBracket);
     builder.endSpan();
   }
 
   visitInstanceCreationExpression(InstanceCreationExpression node) {
     builder.startSpan();
     token(node.keyword);
@@ skipping 1371 matching lines @@
   /// Gets the 1-based line number that the beginning of [token] lies on.
   int _startLine(Token token) => _lineInfo.getLocation(token.offset).lineNumber;
 
   /// Gets the 1-based line number that the end of [token] lies on.
   int _endLine(Token token) => _lineInfo.getLocation(token.end).lineNumber;
 
   /// Gets the 1-based column number that the beginning of [token] lies on.
   int _startColumn(Token token) =>
       _lineInfo.getLocation(token.offset).columnNumber;
 }