Revert "update fasta scanner to always generate INDEX or INDEX_EQ tokens"

pkg/front_end/lib/src/fasta/scanner/abstract_scanner.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.
 
 library fasta.scanner.abstract_scanner;
 
 import 'dart:collection' show ListMixin;
 
 import 'dart:typed_data' show Uint16List, Uint32List;
 
@@ skipping 417 matching lines @@
     } else {
       appendPrecedenceToken(TILDE_INFO);
       return next;
     }
   }
 
   int tokenizeOpenSquareBracket(int next) {
     // [ [] []=
     next = advance();
     if (identical(next, $CLOSE_SQUARE_BRACKET)) {
-      return select($EQ, INDEX_EQ_INFO, INDEX_INFO);
+      Token token = previousToken();
+      if (token is KeywordToken && token.keyword.syntax == 'operator' ||
+          token is SymbolToken && token.info == HASH_INFO) {
+        return select($EQ, INDEX_EQ_INFO, INDEX_INFO);
+      }
     }
     appendBeginGroup(OPEN_SQUARE_BRACKET_INFO);
     return next;
   }
 
   int tokenizeCaret(int next) {
     // ^ ^=
     return select($EQ, CARET_EQ_INFO, CARET_INFO);
   }
 
@@ skipping 792 matching lines @@
       switchToUint32(newLength);
     }
   }
 
   void switchToUint32(int newLength) {
     final newArray = new Uint32List(newLength);
     newArray.setRange(0, arrayLength, array);
     array = newArray;
   }
 }