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 d20a5a3bdc927426b35e945aeec30786157c6fd2..030a2b51f45033a210c18afc5ec82afae794d99e 100644
--- a/pkg/front_end/lib/src/fasta/parser/parser.dart
+++ b/pkg/front_end/lib/src/fasta/parser/parser.dart
@@ -492,6 +492,11 @@ class Parser {
         token = parseOptionalFormalParameters(token, true,
             inFunctionType: inFunctionType);
         break;
+      } else if (identical(value, '[]')) {
+        --parameterCount;
+        reportRecoverableError(token, ErrorKind.EmptyOptionalParameterList);
+        token = token.next;
+        break;
       }
       token = parseFormalParameter(token, FormalParameterType.REQUIRED,
           inFunctionType: inFunctionType);