Introduce element categories and move resolver towards being more compositional.

dart/frog/leg/scanner/listener.dart

Index: dart/frog/leg/scanner/listener.dart
diff --git a/dart/frog/leg/scanner/listener.dart b/dart/frog/leg/scanner/listener.dart
index 2d7e4fb59297dd2c7aad7eadc3a32e5d2158b6f8..78acfdf7d66df458cf07fdd34db92396659a3d47 100644
--- a/dart/frog/leg/scanner/listener.dart
+++ b/dart/frog/leg/scanner/listener.dart
@@ -1222,7 +1222,9 @@ class NodeListener extends ElementListener {
     NodeList arguments =
         makeNodeList(1, openSquareBracket, closeSquareBracket, null);
     Node receiver = popNode();
-    Node selector = new Operator.synthetic('[]');
+    Token token =
+      new StringToken(INDEX_INFO, '[]', openSquareBracket.charOffset);
+    Node selector = new Operator(token);
     pushNode(new Send(receiver, selector, arguments));
   }