Use specific messages for const constructor with body.

pkg/dart_messages/lib/generated/shared_messages.json

 {
   "exampleMessage": {
     "id": "use an Id generated by bin/message_id.dart",
     "subId": 0,
     "categories": [
       "AnalysisOptionsError"
     ],
     "template": "#use #named #arguments",
     "templateHoleOrder": [
       "arguments",
       "named",
       "use"
     ],
     "howToFix": "an explanation on how to fix things",
     "options": null,
     "usedBy": [],
     "examples": [
       "      Some multiline example;\n      That generates the bug.",
       {
         "fileA.dart": "        or a map from file to content.\n        again multiline",
         "fileB.dart": "        with possibly multiple files.\n        muliline too"
       }
     ]
   },
   "CONST_CONSTRUCTOR_OR_FACTORY_WITH_BODY": {

[Johnni Winther, 2016/03/08 09:36:06]

What to do with this entry?

     "id": "LGJGHW",
     "subId": 0,
     "categories": [
       "ParserError"
     ],
     "template": "Const constructor or factory can't have a body.",
     "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();"
     ]
   },
   "CONST_CONSTRUCTOR_WITH_BODY": {
     "id": "LGJGHW",
     "subId": 1,
     "categories": [
       "ParserError"
     ],
     "template": "Const constructor can't have a body.",
     "templateHoleOrder": null,
     "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();"
     ]
   },
   "CONST_FACTORY": {
     "id": "LGJGHW",
     "subId": 2,
     "categories": [
       "ParserError"
     ],
     "template": "Only redirecting factory constructors can be declared to be 'const'.",
     "templateHoleOrder": null,
     "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();"
     ]
   },
   "EXTRANEOUS_MODIFIER": {
     "id": "GRKIQE",
     "subId": 0,
     "categories": [
       "ParserError"
@@ skipping 614 matching lines @@
     "examples": [
       "        main() {\n          final x = 1;\n          x = 2;\n        }",
       "        main() {\n          const x = 1;\n          x = 2;\n        }\n        ",
       "        final x = 1;\n        main() { x = 3; }\n        ",
       "        const x = 1;\n        main() { x = 3; }\n        ",
       "get foo => null  main() { foo = 5; }",
       "const foo = 0  main() { foo = 5; }"
     ]
   }
 }