Use specific messages for const constructor with body.

pkg/dart_messages/lib/shared_messages.dart

Index: pkg/dart_messages/lib/shared_messages.dart
diff --git a/pkg/dart_messages/lib/shared_messages.dart b/pkg/dart_messages/lib/shared_messages.dart
index 5aa2cededb7a5963715c22b46f97fa240d8fe3e8..dc9c9a847d85c58900fe91ccce04a36d5c42425a 100644
--- a/pkg/dart_messages/lib/shared_messages.dart
+++ b/pkg/dart_messages/lib/shared_messages.dart
@@ -198,7 +198,7 @@ final Map<String, Message> MESSAGES = {
       categories: [Category.parserError],
       template: "Const constructor or factory can't have a body.",
       howToFix: "Remove the 'const' keyword or the body.",
-      usedBy: [dart2js],
+      usedBy: [],
       examples: const [
         r"""
          class C {
@@ -221,7 +221,7 @@ final Map<String, Message> MESSAGES = {
       categories: [Category.parserError],
       template: "Const constructor can't have a body.",
       howToFix: "Try removing the 'const' keyword or the body.",
-      usedBy: [analyzer],
+      usedBy: [analyzer, dart2js],
       examples: const [
         r"""
          class C {
@@ -240,7 +240,7 @@ final Map<String, Message> MESSAGES = {
           "be 'const'.",
       howToFix: "Try removing the 'const' keyword or replacing the body with "
           "'=' followed by a valid target.",
-      usedBy: [analyzer],
+      usedBy: [analyzer, dart2js],
       examples: const [
         r"""
          class C {