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

pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart

Index: pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart b/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
index fc6847128953a6720951960340a0fb9110194cbb..165b243b68f8460b30c55bc6c42a90d03814f917 100644
--- a/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
@@ -435,7 +435,11 @@ abstract class AbstractScanner implements Scanner {
     // [ [] []=
     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;