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

pkg/front_end/lib/src/fasta/parser/parser.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.parser.parser;
 
 import '../scanner.dart' show ErrorToken;
 
 import '../scanner/recover.dart' show closeBraceFor, skipToEof;
 
@@ skipping 474 matching lines @@
       ++parameterCount;
       String value = token.stringValue;
       if (identical(value, '[')) {
         token = parseOptionalFormalParameters(token, false,
             inFunctionType: inFunctionType);
         break;
       } else if (identical(value, '{')) {
         token = parseOptionalFormalParameters(token, true,
             inFunctionType: inFunctionType);
         break;
-      } else if (identical(value, '[]')) {
-        reportRecoverableError(token,
-            ErrorKind.EmptyOptionalParameterList);
-        break;
       }
       token = parseFormalParameter(token, FormalParameterType.REQUIRED,
           inFunctionType: inFunctionType);
     } while (optional(',', token));
     listener.endFormalParameters(parameterCount, begin, token);
     return expect(')', token);
   }
 
   Token parseFormalParameter(Token token, FormalParameterType kind,
       {bool inFunctionType: false}) {
@@ skipping 3073 matching lines @@
         break;
     }
     if (isRecoverable) {
       listener.handleRecoverableError(token, kind, arguments);
       return null;
     } else {
       return listener.handleUnrecoverableError(token, kind, arguments)?.next;
     }
   }
 }