report const class as a more specific error code

pkg/front_end/messages.yaml

Index: pkg/front_end/messages.yaml
diff --git a/pkg/front_end/messages.yaml b/pkg/front_end/messages.yaml
index 23e3fd5a40c3b5d73ab442107627543ccac0a156..94cb4153fc619e154cc31e99ea5bdc520c9ce948 100644
--- a/pkg/front_end/messages.yaml
+++ b/pkg/front_end/messages.yaml
@@ -174,6 +174,13 @@ 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'."