Rework type assignability, subtype, and "more specific than" checking.

pkg/analyzer/test/generated/compile_time_error_code_test.dart

 // Copyright (c) 2014, 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.
 
 library engine.compile_time_error_code_test;
 
 import 'package:analyzer/src/generated/error.dart';
 import 'package:analyzer/src/generated/parser.dart' show ParserErrorCode;
 import 'package:analyzer/src/generated/source_io.dart';
 import 'package:unittest/unittest.dart' as _ut;
@@ skipping 5233 matching lines @@
   void test_typeAliasCannotReferenceItself_11987() {
     Source source = addSource(r'''
 typedef void F(List<G> l);
 typedef void G(List<F> l);
 main() {
   F foo(G g) => g;
 }''');
     resolve(source);
     assertErrors(source, [
       CompileTimeErrorCode.TYPE_ALIAS_CANNOT_REFERENCE_ITSELF,
-      CompileTimeErrorCode.TYPE_ALIAS_CANNOT_REFERENCE_ITSELF,
-      StaticTypeWarningCode.RETURN_OF_INVALID_TYPE
+      CompileTimeErrorCode.TYPE_ALIAS_CANNOT_REFERENCE_ITSELF
     ]);
     verify([source]);
   }
 
   void test_typeAliasCannotReferenceItself_19459() {
     // A complex example involving multiple classes.  This is legal, since
     // typedef F references itself only via a class.
     Source source = addSource(r'''
 class A<B, C> {}
 abstract class D {
@@ skipping 408 matching lines @@
         source, [CompileTimeErrorCode.WRONG_NUMBER_OF_PARAMETERS_FOR_OPERATOR]);
     verify([source]);
     reset();
   }
 
   void _check_wrongNumberOfParametersForOperator1(String name) {
     _check_wrongNumberOfParametersForOperator(name, "");
     _check_wrongNumberOfParametersForOperator(name, "a, b");
   }
 }