Use specific messages for const constructor with body.

pkg/compiler/lib/src/diagnostics/generated/shared_messages.dart

 // Copyright (c) 2015, 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.
 /*
 DON'T EDIT. GENERATED. DON'T EDIT.
 This file has been generated by 'publish.dart' in the dart_messages package.
 
 Messages are maintained in `lib/shared_messages.dart` of that same package.
 After any change to that file, run `bin/publish.dart` to generate a new version
 of the json, dart2js and analyzer representations.
 */
 import '../messages.dart' show MessageKind, MessageTemplate;
 
 const Map<MessageKind, MessageTemplate> TEMPLATES = const <MessageKind, MessageTemplate>{ 
-  MessageKind.CONST_CONSTRUCTOR_OR_FACTORY_WITH_BODY: const MessageTemplate(
-    MessageKind.CONST_CONSTRUCTOR_OR_FACTORY_WITH_BODY,
-    "Const constructor or factory can't have a body.",
-    howToFix: "Remove the 'const' keyword or the body.",
+  MessageKind.CONST_CONSTRUCTOR_WITH_BODY: const MessageTemplate(
+    MessageKind.CONST_CONSTRUCTOR_WITH_BODY,
+    "Const constructor can't have a body.",
+    howToFix: "Try removing the 'const' keyword or the body.",
     examples: const [
       r"""
          class C {
            const C() {}
          }
 
          main() => new C();""",
+    ]
+  ),  // Generated. Don't edit.
+  MessageKind.CONST_FACTORY: const MessageTemplate(
+    MessageKind.CONST_FACTORY,
+    "Only redirecting factory constructors can be declared to be 'const'.",
+    howToFix: "Try removing the 'const' keyword or replacing the body with '=' followed by a valid target.",
+    examples: const [
       r"""
          class C {
            const factory C() {}
          }
 
          main() => new C();""",
     ]
   ),  // Generated. Don't edit.
   MessageKind.EXTRANEOUS_MODIFIER: const MessageTemplate(
     MessageKind.EXTRANEOUS_MODIFIER,
@@ skipping 187 matching lines @@
         """,
       r"""
         const x = 1;
         main() { x = 3; }
         """,
       "get foo => null  main() { foo = 5; }",
       "const foo = 0  main() { foo = 5; }",
     ]
   ),  // Generated. Don't edit.
 };