Insert @internal when we generate diagnostic messages.

scripts/processDiagnosticMessages.ts

@@ -54,6 +54,7 @@ function buildInfoFileOutput(messageTable: InputDiagnosticMessageTable, nameMap:
     var result =
         '// <auto-generated />\r\n' +
         '/// <reference path="types.ts" />\r\n' +
+        '/* @internal */\r\n' +
         'module ts {\r\n' +
         '    export var Diagnostics = {\r\n';
     var names = Utilities.getObjectKeys(messageTable);