[strong] Deprecate for-in

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 19d61041d1942c400b9fce07e56ca39a2130d38a..510147f68e2079e6628465afb16d93d001d7d48d 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -164,6 +164,7 @@ var kMessages = {
   strong_equal:                  ["Please don't use '==' or '!=' in strong mode, use '===' or '!==' instead"],
   strong_delete:                 ["Please don't use 'delete' in strong mode, use maps or sets instead"],
   strong_var:                    ["Please don't use 'var' in strong mode, use 'let' or 'const' instead"],
+  strong_for_in:                 ["Please don't use 'for'-'in' loops in strong mode, use 'for'-'of' instead"],
   strong_empty:                  ["Please don't use empty sub-statements in strong mode, make them explicit with '{}' instead"],
   sloppy_lexical:                ["Block-scoped declarations (let, const, function, class) not yet supported outside strict mode"],
   malformed_arrow_function_parameter_list: ["Malformed arrow function parameter list"],