Fix two crashes.

dart/frog/leg/tree_validator.dart

 // Copyright (c) 2011, 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.
 
 class TreeValidatorTask extends CompilerTask {
   TreeValidatorTask(Compiler compiler) : super(compiler);
 
   void validate(Node tree) {
     assert(check(tree));
   }
@@ skipping 25 matching lines @@
 
   visitSendSet(SendSet node) {
     final selector = node.selector;
     final name = node.assignmentOperator.source.stringValue;
     final arguments = node.arguments;
 
     expect(node, arguments !== null);
     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());
     }
   }
 }
 
 class InvalidNodeError {
   final Node node;
   final String message;
   InvalidNodeError(this.node, [this.message]);
 
   toString() {
     String nodeString = new Unparser(true).unparse(node);
     String result = 'invalid node: $nodeString';
     if (message !== null) result = '$result ($message)';
     return result;
   }
 }