fix safari syntax error - decalring twice

fix mishoo#1753.
to mangle names properly - to avoid safari error, scope of for loop should enclose parent scope variables

lib/scope.js

@@ -112,6 +112,7 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options){
     var in_destructuring = null;
     var in_export = false;
     var in_block = 0;
+    var for_scopes = [];
     var tw = new TreeWalker(function(node, descend){
         if (node.is_block_scope()) {
             var save_scope = scope;
@@ -122,6 +123,13 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options){
                 scope.uses_eval = save_scope.uses_eval;
                 scope.directives = save_scope.directives;
             }
+            if (options.safari10) {
+                if (node instanceof AST_For ||
+                    node instanceof AST_ForIn ||
+                    node instanceof AST_ForOf) {
+                    for_scopes.push(scope);
+                }
+            }
             descend();
             scope = save_scope;
             return true;
@@ -303,6 +311,19 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options){
         }));
     }
 
+    // pass 4: add symbol definitions to loop scopes
+    // Safari/Webkit bug workaround - loop init let variable shadowing argument.
+    // https://github.com/mishoo/UglifyJS2/issues/1753
+    // https://bugs.webkit.org/show_bug.cgi?id=171041
+    if (options.safari10) {
+        for (var i = 0; i < for_scopes.length; i++) {
+            var scope = for_scopes[i];
+            scope.parent_scope.variables.each(function(def) {
+                push_uniq(scope.enclosed, def);
+            });
+        }
+    }
+
     if (options.cache) {
         this.cname = options.cache.cname;
     }