Fix two crashes.

dart/frog/leg/tree_validator.dart

Index: dart/frog/leg/tree_validator.dart
diff --git a/dart/frog/leg/tree_validator.dart b/dart/frog/leg/tree_validator.dart
index fab055be705cb6645b2271f819529814b66c1e51..9683721a6b0cb464feef6ece6dfdfa7c717647dd 100644
--- a/dart/frog/leg/tree_validator.dart
+++ b/dart/frog/leg/tree_validator.dart
@@ -43,7 +43,11 @@ class ValidatorVisitor extends AbstractVisitor {
     expect(node, selector is Identifier, 'selector is not assignable');
     if (name === '++' || name === '--') {
       expect(node, node.assignmentOperator is Operator);
-      expect(node, node.arguments.isEmpty());
+      if (node.isIndex) {
+        expect(node.arguments.tail.head, node.arguments.tail.isEmpty());
+      } else {
+        expect(node.arguments.head, node.arguments.isEmpty());
+      }
     } else {
       expect(node, !node.arguments.isEmpty());
     }