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

pkg/front_end/lib/src/fasta/parser/parser.dart

Index: pkg/front_end/lib/src/fasta/parser/parser.dart
diff --git a/pkg/front_end/lib/src/fasta/parser/parser.dart b/pkg/front_end/lib/src/fasta/parser/parser.dart
index 74b9929e2b6d232a57e34869ca6ef749c46b4d6f..d20a5a3bdc927426b35e945aeec30786157c6fd2 100644
--- a/pkg/front_end/lib/src/fasta/parser/parser.dart
+++ b/pkg/front_end/lib/src/fasta/parser/parser.dart
@@ -492,10 +492,6 @@ class Parser {
         token = parseOptionalFormalParameters(token, true,
             inFunctionType: inFunctionType);
         break;
-      } else if (identical(value, '[]')) {
-        reportRecoverableError(token,
-            ErrorKind.EmptyOptionalParameterList);
-        break;
       }
       token = parseFormalParameter(token, FormalParameterType.REQUIRED,
           inFunctionType: inFunctionType);