[front end] Remove type argument from class Code.

This type argument was never used. Removing it should make it easier
to make changes to the front end error message logic (and associated
code generator), which I intend to do as part of a long term plan to
make the analyzer and front end error reporting systems more
consistent, and eventually merge them.

Change-Id: I994cd2a74c5fd77df192f6a1f2dde46ba8a31883
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/442723
Reviewed-by: Nicholas Shahan <[email protected]>
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Johnni Winther <[email protected]>

Author: stereotype441

pkg/_fe_analyzer_shared/lib/src/messages/codes.dart

@@ -18,7 +18,7 @@ part 'codes_generated.dart';
 
 const int noLength = 1;
 
-class Code<T> {
+class Code {
   final String name;
 
   /// The unique positive integer associated with this code,
@@ -42,7 +42,7 @@ class Code<T> {
 }
 
 class Message {
-  final Code<dynamic> code;
+  final Code code;
 
   final String problemMessage;
 
@@ -71,7 +71,7 @@ class Message {
   }
 }
 
-class MessageCode extends Code<Null> implements Message {
+class MessageCode extends Code implements Message {
   @override
   final String problemMessage;
 
@@ -91,7 +91,7 @@ class MessageCode extends Code<Null> implements Message {
   Map<String, dynamic> get arguments => const <String, dynamic>{};
 
   @override
-  Code<dynamic> get code => this;
+  Code get code => this;
 
   @override
   LocatedMessage withLocation(Uri uri, int charOffset, int length) {
@@ -140,7 +140,7 @@ class LocatedMessage implements Comparable<LocatedMessage> {
     this.messageObject,
   );
 
-  Code<dynamic> get code => messageObject.code;
+  Code get code => messageObject.code;
 
   String get problemMessage => messageObject.problemMessage;
 
@@ -245,7 +245,7 @@ class FormattedMessage implements DiagnosticMessage {
     this.involvedFiles,
   });
 
-  Code<dynamic> get code => locatedMessage.code;
+  Code get code => locatedMessage.code;
 
   @override
   String get codeName => code.name;