Introduce element categories and move resolver towards being more compositional.

dart/frog/leg/warnings.dart

Index: dart/frog/leg/warnings.dart
diff --git a/dart/frog/leg/warnings.dart b/dart/frog/leg/warnings.dart
index 8bb94c9301c325409e7213ad1a32dab31dc54f42..197c686bb14d71d7f29e794c07ab31c31e313b7e 100644
--- a/dart/frog/leg/warnings.dart
+++ b/dart/frog/leg/warnings.dart
@@ -46,6 +46,8 @@ class MessageKind {
       'duplicate definition of #{1}');
   static final NOT_A_TYPE = const MessageKind(
       '#{1} is not a type');
+  static final NOT_A_PREFIX = const MessageKind(
+      '#{1} is not a prefix');
   static final NO_SUPER_IN_OBJECT = const MessageKind(
       "'Object' does not have a superclass");
   static final CANNOT_FIND_CONSTRUCTOR = const MessageKind(
@@ -114,6 +116,8 @@ class MessageKind {
       'unused label #{1}');
   static final INVALID_CONTINUE = const MessageKind(
       'target of continue is not a loop or switch case');
+  static final INVALID_USE_OF_SUPER = const MessageKind(
+      'super not allowed here');
 
   static final NOT_A_COMPILE_TIME_CONSTANT = const MessageKind(
       'not a compile-time constant');