Disable classes in sloppy mode unless --harmony-sloppy is set

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 8472454a37c4d99d550e1482f61a62b0559cfb48..142e47bcc0be8b7de3e52fb998fea5597da18cb0 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -180,7 +180,8 @@ var kMessages = {
   unexpected_super:              ["'super' keyword unexpected here"],
   extends_value_not_a_function:  ["Class extends value ", "%0", " is not a function or null"],
   prototype_parent_not_an_object: ["Class extends value does not have valid prototype property ", "%0"],
-  duplicate_constructor:         ["A class may only have one constructor"]
+  duplicate_constructor:         ["A class may only have one constructor"],
+  sloppy_lexical:                ["Block-scoped declarations (let, const, function, class) not yet supported outside strict mode"],
 };