Fix unparse of sendset and add Unparser constructor TODO

lib/compiler/implementation/tree/unparser.dart

 // Copyright (c) 2012, 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.
 
 // Returns null if no need to rename a node.
 typedef String Renamer(Node node);
 
 class Unparser implements Visitor {
   Renamer rename;
   StringBuffer sb;
 
   Unparser() {
+    // TODO(smok): Move this to initializer once dart2js stops complaining
+    // about closures in initializers.
     rename = (Node node) => null;
   }
   Unparser.withRenamer(this.rename);
 
   String unparse(Node node) {
     sb = new StringBuffer();
     visit(node);
     return sb.toString();
   }
 
@@ skipping 241 matching lines @@
   }
 
   visitSendSet(SendSet node) {
     if (node.isPrefix) {
       sb.add(' ');
       visit(node.assignmentOperator);
     }
     unparseSendReceiver(node);
     if (node.isIndex) {
       sb.add('[');
-      sb.add(node.arguments.head);
+      visit(node.arguments.head);
       sb.add(']');
       if (!node.isPrefix) visit(node.assignmentOperator);
       unparseNodeListFrom(node.argumentsNode, node.argumentsNode.nodes.tail);
     } else {
       visit(node.selector);
       if (!node.isPrefix) {
         visit(node.assignmentOperator);
         if (node.assignmentOperator.source.slowToString() != '=') sb.add(' ');
       }
       visit(node.argumentsNode);
@@ skipping 196 matching lines @@
       sb.add(' ');
     }
     visit(node.name);
     if (node.typeParameters !== null) {
       visit(node.typeParameters);
     }
     visit(node.formals);
     add(node.endToken.value);
   }
 }