report const class as a more specific error code

pkg/front_end/messages.yaml

 # Copyright (c) 2017, 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.
 
 # Each entry in this map corresponds to a diagnostic message. Ideally, each
 # entry contains three parts:
 #
 # 1. A message template (template).
 #
 # 2. A suggestion for how to correct the problem (tip).
@@ skipping 156 matching lines @@
 
 ExpectedString:
   template: "Expected a String, but got '#lexeme'."
   analyzerCode: EXPECTED_STRING_LITERAL
   dart2jsCode: "*fatal*"
 
 ExpectedType:
   template: "Expected a type, but got '#lexeme'."
   dart2jsCode: "*fatal*"
 
+ConstClass:
+  template: "Classes can't be declared to be 'const'."
+  tip: "Try removing the 'const' keyword. If you're trying to indicate that instances of the class can be constants, place the 'const' keyword on  the class' constructor(s)."
+  analyzerCode: CONST_CLASS
+  dart2jsCode: EXTRANEOUS_MODIFIER
+  script: "const class C {}"
+
 ExtraneousModifier:
   template: "Can't have modifier '#lexeme' here."
   tip: "Try removing '#lexeme'."
   analyzerCode: EXTRANEOUS_MODIFIER
   dart2jsCode: EXTRANEOUS_MODIFIER
   script:
     - "var String foo; main(){}"
     - "var set foo; main(){}"
     - "var final foo; main(){}"
     - "var var foo; main(){}"
@@ skipping 734 matching lines @@
 PackageNotFound:
   template: "Could not resolve the package '#name' in '#uri'."
 
 InvalidPackageUri:
   template: "Invalid package Uri '#uri':\n  #string."
 
 IntegerLiteralIsOutOfRange:
   template: "The integer literal #lexeme can't be represented in 64 bits."
   tip: "Try using BigInt (from 'dart:typed_data' library) if you need an integer larger than 9,223,372,036,854,775,807 or less than -9,223,372,036,854,775,808."