Report an error if a const constructor calls a non-const constructor.

tests/language/const_constructor_mixin_test.dart

Index: tests/language/const_constructor_mixin_test.dart
===================================================================
--- tests/language/const_constructor_mixin_test.dart	(revision 25201)
+++ tests/language/const_constructor_mixin_test.dart	(working copy)
@@ -14,5 +14,5 @@
 }
 
 main() {
-  var a = const B(42);
+  var a = const B(42); /// 01: compile-time error

[floitsch, 2013/07/19 11:59:45]

ditto. I would prefer the "///" on the `with Mixin` line.

[ngeoffray, 2013/07/19 12:04:06]

Done.

 }