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 bd6fbfd3ede0c1fcf23ee42703d83739cb02cc36..142e47bcc0be8b7de3e52fb998fea5597da18cb0 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -181,7 +181,7 @@ var kMessages = {
   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"],
-  lexical_strict_mode:           ["Lexical declarations are currently only allowed in strict mode"],
+  sloppy_lexical:                ["Block-scoped declarations (let, const, function, class) not yet supported outside strict mode"],
 };