workaround Safari quirks (#4314)

fixes #1753

lib/scope.js

@@ -486,6 +486,7 @@ function _default_mangler_options(options) {
         keep_fnames : false,
         reserved    : [],
         toplevel    : false,
+        webkit      : false,
     });
     if (!Array.isArray(options.reserved)) options.reserved = [];
     // Never mangle arguments
@@ -520,6 +521,21 @@ AST_Toplevel.DEFMETHOD("mangle_names", function(options) {
             return true;
         }
         if (node instanceof AST_BlockScope) {
+            if (options.webkit && node instanceof AST_IterationStatement && node.init instanceof AST_Let) {
+                node.init.definitions.forEach(function(defn) {
+                    defn.name.match_symbol(function(sym) {
+                        if (!(sym instanceof AST_SymbolLet)) return;
+                        var def = sym.definition();
+                        var scope = sym.scope.parent_scope;
+                        var redef = scope.def_variable(sym);
+                        sym.thedef = def;
+                        scope.to_mangle.push(redef);
+                        def.redefined = function() {
+                            return redef;
+                        };
+                    });
+                }, true);
+            }
             node.to_mangle = [];
             node.variables.each(function(def) {
                 if (!defer_redef(def)) node.to_mangle.push(def);