[hack] Clean up comments for expression dependent type tests

Summary: 1. Left a comment in code that I forgot to remove before committing in Naming
2. Changed error message to use a possessive ' (s/classes/class')
3. More descriptive comments in a few tests

Reviewed By: @dabek

Differential Revision: D2250257

hack/naming/naming.ml

@@ -1209,9 +1209,7 @@ and class_prop env x acc =
   | Const _ -> acc
   | AbsConst _ -> acc
   | ClassVars (kl, h, cvl) when not (List.mem Static kl) ->
-    (* there are no covariance issues with private members *)
-    let forbid_this = false (*not (List.mem Private kl)*) in
-    let h = opt_map (hint ~forbid_this env) h in
+    let h = opt_map (hint env) h in
     let cvl = List.map (class_prop_ env) cvl in
     let cvl = List.map (fill_prop kl h) cvl in
     cvl @ acc

hack/utils/errors.ml

@@ -523,7 +523,7 @@ let this_hint_outside_class pos =
 
 let this_type_forbidden pos =
  add Naming.this_must_be_return pos
-    "The type \"this\" cannot be used as a constraint on a classes generic, \
+    "The type \"this\" cannot be used as a constraint on a class' generic, \
      or as the type of a static member variable"
 
 let lowercase_this pos type_ =