Use specific messages for const constructor with body.

pkg/dart_messages/lib/generated/shared_messages.json

Index: pkg/dart_messages/lib/generated/shared_messages.json
diff --git a/pkg/dart_messages/lib/generated/shared_messages.json b/pkg/dart_messages/lib/generated/shared_messages.json
index 7693f4af48e96a14db37f9073b0941052bfdcb67..74457a4505638076384aae74ded94d8179075280 100644
--- a/pkg/dart_messages/lib/generated/shared_messages.json
+++ b/pkg/dart_messages/lib/generated/shared_messages.json
@@ -32,9 +32,7 @@
     "templateHoleOrder": null,
     "howToFix": "Remove the 'const' keyword or the body.",
     "options": null,
-    "usedBy": [
-      "Platform.dart2js"
-    ],
+    "usedBy": [],
     "examples": [
       "         class C {\n           const C() {}\n         }\n\n         main() => new C();",
       "         class C {\n           const factory C() {}\n         }\n\n         main() => new C();"
@@ -51,7 +49,8 @@
     "howToFix": "Try removing the 'const' keyword or the body.",
     "options": null,
     "usedBy": [
-      "Platform.analyzer"
+      "Platform.analyzer",
+      "Platform.dart2js"
     ],
     "examples": [
       "         class C {\n           const C() {}\n         }\n\n         main() => new C();"
@@ -68,7 +67,8 @@
     "howToFix": "Try removing the 'const' keyword or replacing the body with '=' followed by a valid target.",
     "options": null,
     "usedBy": [
-      "Platform.analyzer"
+      "Platform.analyzer",
+      "Platform.dart2js"
     ],
     "examples": [
       "         class C {\n           const factory C() {}\n         }\n\n         main() => new C();"