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,

[ahe, 2017/03/09 12:45:18]

Nice. This should work well with endFormalParameters being called with
parameterCount == 0 below.

The new error may change the status of some tests. Be sure to test -cdartk:

./tools/build.py -mrelease runtime_kernel && ./tools/test.py -mrelease -cdartk
co19 language kernel --time -pcolor --report --failure-summary -j16

You may need to update these files:

tests/co19/co19-kernel.status
tests/kernel/kernel.status
tests/language/language_kernel.status

+            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 3068 matching lines @@
         break;
     }
     if (isRecoverable) {
       listener.handleRecoverableError(token, kind, arguments);
       return null;
     } else {
       return listener.handleUnrecoverableError(token, kind, arguments);
     }
   }
 }